Our story begins in the middle of an argument between two unicorn sisters.
"Sweetie Belle! You can't keep the kitten, you still haven't proved you're responsible enough to care for-"
"But Rarity, how am I supposed to prove I can be responsible if you never give me the chance?"
"Sweetie Belle, don't give me that attitude!"
"Ever since Opal died you've been a jerk Rarity."
"Sweetie Belle! I-"
"AND I WISH YOU WERE A FOAL SO YOU'D BE MY LITTLE SISTER, BECAUSE I'D TAKE MUCH BETTER CARE OF YOU THAN YOU DO OF ME!"
Rarity was stunned. Sweetie Belle had never acted like this before. Rarity decided she needed to be alone. Maybe her younger sister had a point. She did act differently since Opal died one year before. She had also been having these arguments for just as long.
"Sweetie Belle. Go to your room."
"I was already heading there anyways."
Sweetie Belle trotted up the stairs and into her room. Rarity sighed and went into the kitchen to get herself something to drink. Rarity sighed and sipped her coffee.
"Oooh, work has been so awfully slow these days, it's so hard to take care of everything...."
Another sip of her coffee.
"Maybe. . . Maybe I just need somepony to take care of me instead."
Rarity picked up the newspaper and started reading.
"Maybe Sweetie Belle is right. . . Maybe. . ."
Rarity finished her coffee before heading up to her room. Rarity closed the door behind her. Rarity's marehood was getting warm and moist.
"It's that time, is it? Unnnngh, why must I always get so horny?"
Rarity looked around her room and sighed.
"I hate it! I just get so horny, but nothing works!"
Rarity threw herself onto the bed. Then, suddenly, an idea popped into her head.
"...Foal..."
Rarity opened the closet and found her younger sister's old changing table and crib next to her diapers. She had kept these diapers from when her younger sister had a bedwetting problem. Well, to be honest, Sweetie Belle still does have a bedwetting problem, these are just the spares.
She took one of the diapers and levitated it to her flank.
"I think it might actually fit." Rarity mused to herself.
Rarity couldn't believe she was doing this, but she laid herself down upon the changing table and got the lotion and powder out. She readied herself and put the diaper on. She hopped off and looked into the mirror. She blushed as she realized just how much she actually looked like a foal in them.
"I can't believe they actually fit so well!"
Rarity was amazed, but her admiring of herself was cut short by a sudden burning sensation between her legs.
"Oooaaahnnnn.. Oh Celestia, why does this feel good?" Rarity moaned.
She reached down and started rubbing through the diaper, but froze as a sudden jolt of pure bliss ran through her body.
"Oh my stars, this feels amazing!" Rarity rubbed and rubbed until she had her first orgasm in months. Rarity laid there, limp on the bed for a minute or two, before getting up and feeling the warmth in between her legs again.
"I guess it's time for round two then." Rarity levitated one of her sister's old pacifiers from when she was a baby and popped it in her mouth. She suddenly felt very warm inside, like this was where she belonged. And it made her so horny. She kept rubbing but she couldn't bring herself to another orgasm like before. Then she had the thought.
"maybe I should. . . use it." Rarity blushed as she thought about herself peeing in a diaper like a foal, but then she did it. She let the floodgates open and started peeing in her diaper. It felt amazing. Her legs became weak and wobbly from the pleasure, and she fell onto her rump, pushing the pee right up against her body before it was soaked up by the diaper.
Rarity moaned loudly through the pacifier as her body was filled with the rush of these sensations. She started to rub again, this time almost reaching her orgasm before she had another thought.
"Well, I've already peed, I suppose making a mess won't hurt too much." Rarity opened her bathroom door and checked the medical cabinet.
"Let's see, bandages, tampons, pads, painkillers, medical tape, cotton balls, q-tips, ah-ha! Liquid fleet enemas!"
Rarity took two bottles of the enema and went back into her room. She carefully undid the tapes on her diaper so as not to damage them, and then bent over, sticking her hind quarters into the air. She then inserted both bottles of liquid enemas before taping the diaper back up, this time a little bit tighter. Rarity's intestines growled at her.
"oohh, Rarity, this is so uncouth. . . but I NEED IT SO MUCH!" Rarity stood and waited for the moment her bowels would no longer be able to hold back what's inside. Her hind legs started to spread apart slightly. Then, rarity got her wish, ash her bowels released with a mighty force bother her enemas and a decent sized load of manure into the diaper. Rarity's body shook with pleasure as her legs gave out, forcing the warm mess to make contact with her body. The diaper was tight enough that when she pooped, the mess went all across to the sides of the diaper before making its way down towards her vagina. The mess filled her diaper so much that it reached her clit, causing Rarity to start panting as the pleasure built and built. She was too out of breath to moan as she climaxed, falling over limp with the pacifier in her mouth. With the last of her energy, she lifted herself into her sister's crib and pull and blanket over herself before her eyes grew heavy and shut.
Rarity pulled out some wooden bars from her closet and set them to the sides of her bed, turning it effectively into a giant crib.
"Next step."
Rarity pulled a large mare-sized changing table from the closet as well and hopped up onto it.
"Oh Rarity, you're such a naughty mare." Rarity mused to herself as she levitated one of the diapers onto the table.
Rarity put the diaper on and taped it up nice and tight.
"hhnnnn, why does this feel so good?"
Rarity touched her hoof to where her marehood was and started rubbing, letting forth a few moans.
She pulled back and got off the table, taking a bottle with her.
She got some formula and mixed it with the bottle before putting in her own microwave in her room to heat it up.
While she waited, Rarity got out the rest of her foal toys and stuffed animals.
The microwave beeped.
Rarity took the still warm oatmeal and dumped it down the seat of her diaper, making sure not to stretch it too far as to make it slip off. She then took the bottle, laid herself down onto a pile of stuffed animals, closed her eyes, and started sucking while rocking herself.
Her face was getting incredibly red. Even when nopony could see her she was still embarrassed that she could relax so much in such a way.
Rarity felt her bladder get full while drinking, so she let it go. She let another moan out as her wet and full diaper rubbed against her body while she rocked herself while drinking.
Rarity finished her bottle and burped herself, the oatmeal in her diaper still somewhat warm and mushy.
Rarity was happy as she was.
But then her smile faded as she remembered what her sister had said earlier.
Rarity went to her mini fridge and took out a potion she had made long ago.
A potion to regress a pony's mental state into that of a foal.
The effects were permanent and irreversible.
They would stay as a foal for the rest of their lives.
Rarity pressed her hind quarters into each other and moaned.
She'd never felt this horny in her life.
Her stomach gurgled.
She knew what she had to do now.
Arguing to herself over the potion would have to wait, she needed to drop a massive load.
Rarity squatted and grunted and forced a massive amount of poop from her rear end.
There she was, the formerly successful fashionista and clothing designer, now in her room, in a very wet, and very full diaper.
She sat down and felt the mess mush her backside.
Rarity couldn't hold back a loud audible moan as the oatmeal and manure squished all over her rear and over her privates.
Rarity took the potion, panting from the sexual pleasure she was being overcome with.
"Once I drink this, I can't go back. I'll have 30 seconds before it takes effect."
Rarity let a tear fall from her eye.
"Sweetie Belle, you're going to prove you're responsible to me, but you only have one chance."
Rarity drank the potion.
"Sweetie Belle, my dear little sister, I'm so sorry. I'm so sorry that your older sister, the one who should have been taking care of you, is nothing more than a little foal who can't even look after herself."
Rarity then threw the bottle into the trash bin, put a large light blue pacifier, obviously custom made for adult ponies, into her mouth, and started to rub her diaper furiously.
Rarity's moans were loud and her pleasure was beyond anything she had ever felt before.
It had been 25 seconds.
Rarity kept rubbing herself and sucking her pacifier in rhythm with her movements.
And then, overcome with pleasure, Rarity let out a loud yell of pure pleasure as she had the greatest climax anypony in Equestria had ever felt.
And that glorious moment would be the last thing Rarity would ever know.
"Rarity?"
Sweetie Belle opened the door to Rarity's room.
"Rarit-... Is.. Is that my diaper?!"
Rarity just made a little spittle and sat there, staring into space.
"Rarity?"
Sweetie Belle moved closer. When she didn't get a normal response, she started to cry.
"Rarity! Oh Celestia, what did you do?"
Sweetie Belle hugged her "little" sister and cried, knowing that things would never be the same.
"Don't worry, Rarity. I'll take good care of you. I'll be the best sister anypony could have."
Sweetie Belle sobbed into the rest of the night.