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Guinea Pig
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The sun shone brightly down on the blue pegasus as she flew through town. The air whipped against her features, it pushed against her wings and howled in her ears. She always enjoyed this feeling of being alive. Her heart beat rapidly as she pumped her wings faster and dipped below the roofs of the town. The adrenaline ripped through her veins as she tore passed pony after pony, kicking up a cloud of dust in her wake that spiralled and folded upon itself. She ignored the protests of the ponies that walked the streets; she wasn't low enough to collide with them anyway. Faster, she drove her wings. She sped like a dart through the air, completely in control. She closed her eyes briefly to delight in the feeling of the pure speed she's maintained.

"Watch out!" A cry pieced her reverie.

She quickly opened her eyes to find that she was about to crash into a pony attached to a wagon; the specifics were lost in the 'I better not crash into this' moment. Quickly, she jutted to the side, narrowly dodging the pony and the wagon of who knows what, probably also causing the unlucky pony to acquire a new hairstyle. She ignored the annoyed sound as she looked back at the obstacle she dodged.

'Heh... that was a close one' she then turned her head back forward to become face to face with a giant purple crystal tree. "Oh buck!" She was too close to slow down, so she quickly covered her face with her hooves to brace some of the blow; maybe she could break her forearms instead of the rest of her body. She tightly scrunched her eyes and clenched her jaw, bracing for the impact that she won't likely win.

It never came.

Cautiously, she opened her eyes. A purple haze overcame her vision as she levitated there, inches away from the solid crystal wall. She let out all the air in her lungs as she breathed a sigh of relief. She looked around to try and find the source of the purple haze. The sound of the front door opening caught her attentions.

Twilight Sparkle appeared in all nerdy princessy glory. Their eyes quickly met as Twilight pursed her lips. "Rainbow Dash,” she said, her tone more of a statement than a question, “what are you doing?"

"Uh... Trying not to die?" Rainbow shrugged as Twilight picked her up in her magic and lowered her to the ground.

"I would really have to assume the opposite judging by the way you were hurtling towards the castle," she shook her head as she placed her friend on the ground, "if it wasn't for my passive acceleration dampening array I placed around the castle, you'd be a Rainbow pancake."

Rainbow winced at the implications. She reached up behind her mane to scratch at the back of her head in an embarrassed manner, "Yeah... Sorry about that. What was that anyway?"

Twilight easily followed the segue, always excited to explain her projects, "Since you're interested, it is a passive acceleration dampening array, designed to continuously cover a certain area with two different types of fields, the primary deceleration array..."

Rainbow Dash nodded along, but quickly began to tune out the sound of Twilight's voice. Sure giant fields are impressive or whatever, but all this magical gibber gabber just flew right over her head. Twilight was so dug into explaining her force fields that she didn't notice Rainbow's disinterested wandering eyes.

Her eyes fell towards Twilight's wings, which hid saddlebags filled with stacks of papers. Interested in these papers Rainbow lifted a hoof to interrupt her lecturing friend, "Hey Twilight, whatcha got there?" She pointed to Twilight's saddlebags.

"What? Oh!" In her excitement of speaking about her magical discoveries, she had momentarily forgotten that she had something she needed to do, which included handing out the papers in her saddlebags, "these are flyers I'm going to hoof out to ponies in the town, I need a volunteer to test a new potion I have developed," She activated her magic to pull out one of the flyers, levitating it over to Rainbow, who quickly read through it once she got a hold of it, "It involves a lot of pain, so I don't expect many ponies would want to go through with it." Once Rainbow had taken the flyer, Twilight moved passed her to go into town.

"Ah huh... potion... magic... testing... blah blah blah," Rainbow tilted her head after she had finished reading, as if in thought, after a moment she shrugged, "Eh, I'll be your volunteer."

The sound of Rainbow's voice stopped Twilight in her tracks, she turned her head to look at Dash, "Really?"

Rainbow shrugged in response, "Yeah, sure, why not? I mean, I'm not doing anything, so I'm both free whenever and really bored," she flicked her head in the direction of the castle, "the only thing I've got going on is crashing into buildings, what do you need me to do?"

Twilight considered her friend for a moment, ‘as it turns out, willing test subjects do fall out of the sky,’ she thought with a smile. She looked back at Rainbow, "Thanks Dash, all I need you to do is come to the castle tomorrow at about three in the afternoon. You don't need to bring anything with you. I'll explain more once you're here."

Twilight then walked passed her and back into the castle. "Okay then, see ya tomorrow," Rainbow said.

"See you tomorrow, Dash... and thanks again!" Twilight replied as she closed the door behind her and disappeared into the castle.

Rainbow looked down back towards her flyer; she briefly scanned it for more details but quickly grew bored. She shrugged and flicked the paper behind her. She'll know more about it tomorrow when Twilight tells her about it, 'I mean, it's not going to be as bad as crashing headfirst into a giant mother bucking tree,' she thought, shaking her head, 'that's for sure.'

She flapped her wings to get in the air, and flew off to the fields outside of town. She may have an obligation tomorrow, but that's a whole day away, she has plenty of time for her to practice her not-flying-into-things flying.

.o0O0o.

Rainbow awoke bright and early the next day, which means she just managed to wake up before the afternoon. Such was this a rare occurrence that she actually managed to get her weather management done for the day, before her boss had to wake her up, surprising both the stallion and her colleagues.

All in all, this started out as a pretty good day for Rainbow. She managed to get her work done, so the sky was nice and clear with a smattering of clouds scattered about in preparation of the storm scheduled in a few days. Rainbow decided that the rest of the afternoon could be spent relaxing as opposed to being reckless like she usually is.

Besides, she wanted to make sure she was prepared for whatever Twilight had planned. She couldn't remember exactly what she read on the flyer, only the words potion and pain floated around her mind as lazily as the cloud upon which she currently rested.

Rainbow sighed as she relaxed further into the cloud she was laying on. She didn't want to fall asleep right now, as she didn't want to miss the deadline Twilight gave her, but the cloud was so soft, and the sun shone so pleasantly on her fur, warming her to the core. It became a real struggle to keep her eyes open.

Eventually, her eyes did slide to a close, her mind succumbing to the relief of not having to focus on anything in particular. The breeze played against her coat as it pushed on and danced with her cloud. It cooled her down to nicely contrast the warming sensation of the sun. She really didn't want to, but she was losing a very steep uphill battle against falling asleep. Her eyes dipped and her body relaxed, letting go of all the tensions she picked up when working. It was such a nice day today, and she should know, she made it that way.

Then she made the mistake of rolling over.

The clouds in the sky weren't big at all, hence 'smattering of clouds', and in her blissful journey into sleep Rainbow had forgotten that the cloud she was resting on was tantamount to a beanbag. So, as she tried to relax further and roll onto her stomach, she simply rolled and slid off the cloud.

Rainbow let out a quick yelp as she was rudely shocked awake. Her reflexes quickly activated, allowing her to right herself and begin flying. A shot of adrenaline spiked through her system, stinging her body from the inside. It ensured that she was woken up completly, if the fall didn't do that already.

She let out an annoyed groan as the adrenaline subsided, then shook her head to try and rid her mind of the pre-nap fog that had failed to take over her. She rubbed her eyes as she looked up to the sun to see if she could guess what time it was. Judging by its position in the sky, it was about two in the afternoon. She shrugged, "I might as well make my way to Twilight's," she said before setting off into a dive in the general direction of the castle.

She didn't push herself too hard, she just sped up enough to hear the wind softly cry in her ears and simply glided to the castle. It was only a few minutes of flight until she touched down outside the front door of the crystal tree and in front of the two guards posted to watch the door.

"Welcome to the castle ma'am, Princess Twilight isn't expecting you for another hour," the guard stated upon the sight of the prismatic pegasus.

"Eh, I figured I'd come around early," Rainbow shrugged it off and moved passed the guards. They parted their purple coloured spears and allowed her entrance. Her hooves echoed around the room as she walked along the hard crystal floors. She found herself wondering where Spike was, usually the little tyke was available to greet guests as they come to the door, no matter what he was doing.

'He must be out on an errand or something...' she then stopped in her movements, and looked around the massive empty halls of the crystal castle, 'where did Twilight want me to go again?

"Rainbow Dash!" Twilight's voice called from behind her, she overshot where she was supposed to go, Twilight was looking at her in wonder, "I wasn't expecting you so early."

Rainbow shrugged, "Eh, you know me, excited for science and stuff."

Twilight looked her up and down, before simply stating, "You fell off something you were napping on didn't you?"

Rainbow raised her eyebrow inquisitively, "Yeah, how did you know?"

"You're not as animated as you are usually, and you just said you were interested in science," Twilight shook her head and smiled, "which means you're tired and delusional, that and you just told me."

Rainbow pursed her lips, "You got me there."

Twilight rolled her eyes and motioned with a wing for Rainbow to follow. They walked for a bit; their doubled rhythmic hoofsteps gave some form of percussion to the empty halls of the crystal castle. The tattoo just bounced off the walls and danced with the newer sounds, all this just added to the stark emptiness of Twilight's castle.

Once they reached a seemingly random door, not far from where they started, Twilight opened the door and let Rainbow trot down the stairs behind it. Twilight then spoke, "I just have to finish setting up a few recording instruments then we can get started."

"Sure, how long should that take- Woah!" Rainbow's exclamation broke her sentence as the full room came in to view. This lab wasn't just different from the old one; it was massive! There were so many different machines scattered about in an orderly fashion, they were all shut off of course, but Rainbow could imagine all the sounds and lights that would fill the room if they were turned on. She didn't realise she was standing at the bottom of the stairs staring on in shock until Twilight broke her daze.

"You should have seen how I reacted when Princess Celestia informed me of my scientific allowance," as Twilight moved passed Rainbow, she softly nudged her with a wing to snap her out of her stupor. She pointed in a direction with the same wing, "Come on Rainbow, follow me, the testing area is ahead."

Rainbow nodded her head, letting the overwhelming feeling of all these machines pass her by. As they traversed through the metallic forest of machines, Rainbow briefly remarked on the different kinds of machines there were. There was one that looked like a microscope, except on steroids, another that looked like one of those super cool plasma ball things, and one that just looked like a refrigerator.

The sight of the refrigerator looking contraption confused her, "Hey, Twilight," she said to her friend, stopping their motion briefly, "what's that machine there?"

Twilight looked at Rainbow incredulously, then she smirked. She cleared her throat and explained exactly what that machine was, "It's a sealed container that uses a multi-cooling infused sapphire array to condense and evaporate the liquids inside, providing the items within a longer life before nutritional decay." Twilight then walked up to it, she put her hoof on the handle and pulled it back to open it.

A tiny delicate wave of coolness touched at Rainbow's hooves. She frowned once she realised what it was.

"It keeps my food cold."

"I know what a fridge is, gees, I was just... never mind."

Twilight laughed and shut the fridge door, "I'd offer you a sandwich, but I'm not sure that's wise." She continued walking, just to the door they were now close to. "Alright," Twilight began, falling into her scientist mode, "this is the room in which the experiment will take place, please enter and wait there for further instructions."

"Sure," Rainbow simply said.

"Oh, and I need this," Twilight gave little warning before she magically pulled a crimson strand of the pegasus' mane out of her head.

"Hey- ow! What was that for?" Rainbow raised her voice as she reached up se she could feel where the pain originated. Her reaction was more just to the shock of it than the actual severity.

"It's to finish off the potion, I just needed a strand of your DNA to complete it."

"A little warning would've been nice," she frowned, but moved passed Twilight to wait in the room. Twilight stuck her tongue out as she closed the door behind Rainbow. The room itself was very basic, the walls were bland and there were only two doors. The one she entered through and another one with the words 'Authorised Personal Only' printed in plain font on it. The floor also seemed to be slightly cushioned, it gave way slightly as Rainbow stepped, yet it didn't feel any different from the floor she was standing on before. It was some form of soft tile? Rainbow didn't know, nor care.

It was a short while before Twilight reappeared through the Authorised Personal door. She was wearing a white scientist cloak with safety goggles on her snout, she was also levitating a vial with some strange liquid in it. It was a very bright blue liquid, probably bluer than Rainbow's coat when the sun hit it directly.

Rainbow was about to ask her what it was before Twilight spoke up, still in her scientist mode, "Thank you for your participation in this experiment, would you like to know more about the experiment before we continue or are you well aware of the risks and dangers of this procedure?"

"I'd uh... like to know more please," Rainbow answered.

"Right," Twilight nodded, "what I currently hold in my magic is a vial containing an experimental potion designed to use a magical matrix to reconfigure the body of the drinker based on a modified strand of the user's DNA."

Rainbow imagined hearing several small explosions of sound; because the terminology flew over her head so fast it broke the sound barrier, twice. "It's a what to do what now?" was her intelligent response.

Twilight smiled at Rainbow's question, "It's a gender swapping potion."

Rainbow raised both her eyebrows in surprise and interest, "Really? So this will make me into a stallion?"

"Correct," Twilight nodded, "before I give you this to drink however, I need to inform you of the effects, risks and warnings about the potion."

"Shoot."

"First and foremost, this procedure involves a lot of pain, the potion sets about reconfiguring your bones and muscles, which also includes warping your nerve endings in your body, leading to intense pain," she waved the potion as if to acknowledge it, "this will also lead to your bones straining your muscles, which will cause them to become very sore afterwards."

Rainbow considered the dangers, before shrugging, pain wasn't something she was new to.

"Once you ingest the potion, it will sting your throat and head before causing you to become very disorientated and dazed, this is expected, the first thing it sets about changing is the chemical make up of the brain for the preliminary changes. It will then begin changing the physical aspects of your body, starting with the head and ending down at your flank," Twilight locked eyes with Rainbow, making sure the next part of her speech was seen as completely serious, "this is the start of the transformation process, which will cause a lot of, if not, tremendous pain. After hearing about these risks, do you still want to proceed with the experiment?"

Rainbow thought about her answer, on the one hoof, excruciating pain sucks big time, but on the other hoof, she's always been a bit curious about what it would be like to be a stallion, she had a small part of her brain wondering what it felt like to be the one entering, as opposed to being the one entered. She shrugged once more, having made up her mind, "Sure, hit me with it."

"Alright, allow me to set up various recording devices so I can monitor your vitals," Twilight then levitated three different objects into Rainbow's sight, one that looks like a headband, one that looked like it fits on her wrist and one that looked like a necklace. Each of them had a certain colour of crystal embedded in it, and the crystals all had a sheen to them that gave off their state of being enchanted. Twilight then put the bands where Rainbow had originally believed they would go, the wrist one levitated to her front right hoof, the necklace came apart so it can wrap around her neck and be secured easily, and the headband simply slipped over her head.

Once everything was secured, they all seemed to let off an audible beep of some kind, likely to let Twilight know that everything was working well. Once she was sure that they were in place and collecting data, Twilight levitated the vial of transfiguration potion to Rainbow's hoof. She then moved to one of the corners of the room, "Once the transformation stages begin, your muscles will seemingly begin to spasm, causing you to thrash about, I will be here to make sure you don't injure yourself too badly. Whenever you're ready Rainbow, you may drink the potion."

Rainbow nodded, then she used her teeth to remove the vial's lid. She briefly let air into her nose, causing her to smell an odd sent of vanilla mixed with blueberries or something. "Welp," Rainbow said, "bottoms up." She tipped the vial into her mouth and drank the potion.

As soon as the potion hit the back of her throat, it began to sharply sting. She couldn't help but cough as the stinging feeling overwhelmed her mouth and throat. The stinging then transformed into a headache as her brain felt like a flood of fog was rolling in. She blinked, and the world seemed to defocus around her, she moved her eyes, but they wouldn't lock on to anything. A smear of purple flew across her vision as she vaguely remembered Twilight's position.

"Woow... You were right," Rainbow slurred her speech as she slowly became unconnected with the world, "this does feel w-" she coughed again, her brain, head and throat suddenly lit up in agonising pain, as if her head had just combusted. She let out a strained yelp as the pain overcame her mind and upper body. The pain then quickly spread down to the rest of her body, lighting up her nerves with fire. She no longer had full control over her muscles, so she fell to the floor. In the period of three seconds after her speech was interrupted, she was on the floor, writhing in agony.

Twilight had to work extra hard to make sure that Rainbow didn't do any permanent damage to her body before it could finish changing. As she thrashed about, Twilight used her fine control of telekinesis to make sure that Rainbow's movements were slowed, so that if she were to swing her foreleg to the floor, instead of breaking the bone due to the speed, it would just press against the ground. It took a good majority of Twilight's concentration to keep her from hurting herself, she was thankful that she had the foresight to enchant the floor to make it more rubberised, otherwise Rainbow would have hurt herself very badly by now.

Then, an aching pain unlike anything Rainbow had felt before or was currently feeling entrapped her head. The transformation stage had started. Sounds of bone creaking and warping filled the pegasus' mind, as her nose, jaw, and cranium began to feel especially painful. They felt like they were moving on their own, but also like they were being constricted by her spasming face muscles. Twilight's telekineses moved into Rainbow's jaw, to hopefully prevent the strength of her clenching jaw from breaking her teeth.

Her facial features had stopped changing, leaving the muscles unable to function, her eyes and mouth dropped loosely and uncontrolled, she couldn't move those muscles, but she was still very much awake and aware of the changes that are now happening to her chest and forelegs. She felt like she was breathing in, but the breath moved way passed what she should have been able to do, she felt almost like her chest was inflating like a balloon. Her ribcage was warping and re-configuring itself to show a more powerful shape, it became more bull-like as opposed to swan-like. Her legs followed along with this change, slightly growing longer and thicker. Becoming stronger and more stallion-like to add to the bull-likeness of her chest. They finished their changes and became limp, the muscles having used up all their energy.

Once her chest and legs stopped changing, her back started cracking. Each of her vertebrae began to push against one another as they expanded and straightened, allowing for the ability to bolster a larger frame. After her spine, her wings were next. They began to strain themselves, twisting into painful positions as the muscles inside them thrashed about. Her fragile wing bones began to grow, spreading out further and growing thicker. Allowing her new wings to be able to hold a larger body in the sky.

Her flank and hindlegs then joined in on the changes, following her front legs and chest in growing longer and thicker, and less mare-like. It lost its petite frame and swan-shaped curves as it grew slightly to match the rest of her body with her now stallion-esque shape. Once her flank and hindlegs finished their changes, her crotch began to feel very painful.

Her teats felt strained, they felt like they were getting pulled as they shrank and were replaced by skin no different from the rest of her body. The following sensation was even stranger, pain flared up in her lower body where her ovaries were as they crushed and reformed, travelling down her body to just at the end of her closing vagina.

She could feel that skin begin to stretch and grow as the eggs that were once inside her ovaries began to repurpose themselves into sperm producing proteins. They separated into two distinct feelings in her crotch. When they started to grow, they began as no bigger than the size of peas. The skin that was covering them stretched, and became a darker colour, letting the growing testicles fall and sit in its hammock like form.

The growth wasn't stopping, her new testicles pushed against the skin, spurring on another stretch to hold them better. After a few more spurts of this, the growing began to slow as the last of her ovaries found their way into her new gene containing orbs. The skin holding them sagged once more, stretching to better hold the impressively sized additions.

Then a mix of overwhelming pain and pleasure flared up from her now-closed vagina. The feeling amplified as her clitoris shifted to just above the testicles and began to expand. It grew outwards and became longer. The appendage added inch after inch, then began to add thickness to itself. The exposed skin on her elongated clitoris became slightly harder and less sensitive to form what more resembles a stallion's penis. It hasn't stopped growing, it got thicker, causing the pleasure to increase passed the threshold of pain, Rainbow's wails of agony quickly shifted to groans of pleasure mixed with pain.

She had only then realised that sometime during her thrashing about, she landed on her stomach, she became aware of this through the kiss of cold flooring on her sensitive growing addition. She could feel it pressing against the floor, making it sink while pushing herself up as it increased in thickness and length. The muscles inside her new penis flexed and ached as the last of its growth pulsed and gradually slowed. She was left on the floor, panting with exhaustion.

Then, as quickly as the pain started, it vanished, and Rainbow was left on the floor. He was dead to the world. The full implications had yet to hit her tired mind. She just preferred to sleep over realising that she is now a he in many aspects. Her breathing slowed as he gave in to the misty-unawareness that is her slipping consciousness. She let her pulsing heart be his metronome, a beat she could use to drift off into a blissful serenity.

The world faded around her as he drifted off to sleep.

.o0O0o.

An uncomfortable, yet not unwelcome feeling came over Twilight as she had witnessed her friend's growing stallionhood. She imagined the feeling of that gigantic phallus penetrating her over and over, causing her mind to reel in imagined pleasure. She really wasn't expecting it to be that large. The slowly silencing scientific section of her brain drew the probable conclusion that Rainbow's latent magical energy could be heightened due to her connection to the Elements of Harmony.

The quick deduction snapped her mind back to the task at hoof and away from lust filled fantasies involving a certain newly formed stallion. She shook her head to clear away the last of the clouds in her mind and moved to pick her friend up in her magic, removing the recording devices as she did so.

Stallion in magical tow, she made her way out of the laboratory and down the hall of the castle to get to the guest bedroom. She gently placed Rainbow in the bed and tucked her in comfortably. Her exhaustion letting her continue her blissful rest undisturbed through the movement, as her new body recovered from the painful transformation.

Twilight looked at the form of her exhausted friend, and noted the complexity and size of her new body. As pleasurable as being penetrated relentlessly by an enormous cock might be, Twilight had something more orgasmic to set her mind to; finalizing the experiment evaluation! So with a flick of her horn, she summoned her designated clipboard with the potion notes already inscribed. She then levitated one of the quills from her lab coat and set to work outlining the results.

Rainbow, was a very good guinea pig.

Author's Note

This may look familiar to a few of you, that's because it is. This is a complete rewrite of my original Transgenderised story. The site rules allow for complete ground-up rewrites as new stories, so it's a new story.

Anyway, so, this is the rewrite wherein Dash is turned into a stallion cause can... and I'm sorry, but I lied to you when I said I'll keep it at 3600 words.

Oh yeah, and here is a deleted scene if you like:

Rainbow Dash nodded along, but quickly began to tune out the sound of Twilight's voice. Sure giant fields are impressive or whatever, but all this magical gibber gabber just flew right over her head. Twilight was so dug into explaining her force field thingy that she didn't notice Rainbow's disinterested wandering eyes.

Her eyes fell towards Twilight's sides, upon which laid saddlebags filled with stacks of papers. Interested in these papers, Rainbow lifted a hoof to interrupt her lecturing friend, "I have just one question to ask you." Rainbow closed her eyes, reared up on her hind legs, fell backwards into a flip, then quickly darted over to Twilight's saddlebags, "WHAT ARE THOOOOOOOOOOOSE?!" She yelled, pointing at her saddle bags, "What are those?"