Twilight jumped. A low hum-like sound resonated through the Everfree Forest, and when a shrieking bird call sounded through the haunted wood, the lavender mare spooked.
"It's going to be okay, it's just a bird, Twilight," she said aloud. "We're just going to find this bird--" Her talking was abruptly cut off as she tripped and fell, landing face-first on a bed of strange luminescent moss. She sat up, spitting dirt out of her mouth. She looked around.
The cave she had fallen into was very large, at least two dozen ponies could have fit in the space. The dark grey stone walls were lit by a mysterious blue light emanating from a large pond in the back of the cavern. Looking up, she realized that the hole she had fallen into was far too high to walk back up, and her magic was suddenly and inexplicably exhausted. Try as she might, she could barely produce the faintest glimmer of light with her horn. Luckily, the moon coupled with the odd light was enough to see.
Twilight got up onto her hooves, walking over to the strange pond. She peered down over the edge of the still, blue water. A single bubble rose to the surface, then popped. She prodded the surface with her hoof, and was surprised to see that it came away dry. Instead of rippling like water would, however, the liquid simply shook slightly, as if it were a gelatin. She poked it again, sinking her hoof deeper into the substance. When it was about three inches in, she tried to pull her hoof back up. But the goo would not let go of her leg, wrapping itself farther up her cannon bone and passing her knee.
The mare pulled with all her physical strength, yet her efforts were in vain. The substance simply enveloped her leg more until it reached her shoulder. Whimpering, the exhausted unicorn lay down, determined not to let herself be taken. The gelatin had other ideas. Twisting itself even further up her left side, the substance continued until it reached her withers before covering her other side. Soon, nearly her whole body was coated in the blue goo.
Adrenaline now flowing through her veins, she tried as hard as she could, but simply could not summon a spell to save her. All she succeeded in producing was a slight electric shock that stopped the tentacle-like goo that was approaching her head, if only for a second. She relaxed, giving up the rather useless fight. However, when her head was the only portion of her body that was not covered, the gelatin parted her mouth, a single strand of it flowing into her mouth. Triggering her gag reflex, she coughed, trying to expel the foreign... thing that had just invaded her mouth. No such luck however, as the tentacle did not react, rather preferring to continue to slide down her throat and into her stomach.
Simultaneously, the goo coating on her body concentrated itself on her hooves and tail area, keeping her immobilized. The blue gelatin underneath her tail grew adventurous, forming small tendrils that teased at her clit, the cold of the goo shocking her. Her eyes widened and more and more of the substance dipped inside her marehood, which was, against her will, dripping with arousal slightly. She tried to scream, tried to yell, but the goo in her mouth prevented that quite well.
As the tentacle-like gelatin in her mouth wriggled around her substance, the pond feeding the tendrils bubbled, and a pony-sized blob of it poured onto the cave floor. She watched as the blob bubble itself, forming into the blue, translucent mirror image of herself, with one minor exception. As the gooey Twilight walked forward, the real Twilight choked on the tentacle in her mouth, seeing the very erect stallionhood in between Goo Twilight's legs.
The tendrils playing with her clit and marehood slid out, instead toying with her small teats. Twilight whimpered loudly as the goo form of herself walked behind her, rubbing her slit. She moaned in pleasure as the goo pony licked her clit, wrapping around it and tugging slightly. It felt so good, yet she did not want it to. Goo Twilight flicked it's tongue deeper inside her slit, sliding in partially. Twilight gasped through the tentacle in her mouth, pleasure and the instinct to breed overwhelmed her senses, slowly taking hold as the goo pony continued teasing her. She moaned as her replica pulled out and paused.
Lifting her tail to reveal her dripping marehood, Twilight moaned again, this time in desperation. She needed to feel that cool, wet goo pony stallionhood sliding in her, filling her passage.
"Fuck me," she said, though it sounded more like "Ucc ee", with the tentacle in her mouth. "NOW!" The goo pony seemed to get the message, as it mounted her. Sliding in only slightly at first, the goo pony teased her, moving it's hips only minimally backwards before going back in. One of it's forehooves came down to brush her clit, making her pant and moan even more. As if Goo Twilight could sense her depraved desperation, it slid out of her marehood all the way, before ramming back in, hitting her cervix. Pulling back out slowly, the goo pony began pistoning in and out at a medium pace, speeding up every few seconds.
The tentacle in her mouth became bored, wriggling around her throat. Shortly, a blob appeared in the tentacle, containing a darkerblue liquid. Though her mouth was already stretched to nearly it's limit, Twilight still swallowed the mass. The tentacle of gelatin deposited it in her stomach, the cold shocking Twilight just a bit. More globs of the liquid appeared, eventually feeding a constant stream of it into her slowly swelling belly. With the pistoning motion from the goo pony in back, her stomach jiggled.
The tendrils that had abandoned her teats returned, small slivers of the goo inserting themselves into her nipples. Slowly, the tentacles in her teats began to expand as more goo filled her mammary glands. They swelled quickly, pushing her back legs apart. The goo pony was still pistoning in and out of her slit, and showed no sign of stopping, despite the fact most stallion would be near climax. Twilight, against all rational thought left in her brain, began bucking her hips in time with Goo Twilight's thrusts. She threw her head back in ecstasy and pleasure as she moaned as her orgasm hit her.
Sensing her orgasm, the goo pony released the deep blue liquid, which Twilight had not noticed gathering in it's body. The pond fed more and more of the liquid into her womb as her stomach and teats, which were still expanding, lifted her off her hooves. Instead of relinquishing the grip the tentacles had of her, the goo pony melted into a new tentacle, still shooting the dark blue substance into her womb. She swelled, the tendrils in her nipples and mouth pulling out to meld with the one in the marehood. Twilight, exhausted from the intense waves of pleasure, simply passed out, the tentacles still feeding her slit.
Twilight awoke the next morning in the mysterious cave, looking nine eleven months pregnant with decuplets. The weight in her teats was enormous, the milk-filled mammary glands still too big to allow Twilight's hooves to touch the ground.
Suddenly, a contraction woke her fully. She was on her side, and as soon as she realized what was happening, another contraction shook her. She had read about the pain of childbirth, not to mention heard about it from various ponies. However, she did not feel pain at all, rather, she felt the sensation of cool liquid sliding out of her marehood. As it exited, she could see a small purple foal. With a start, Twilight realized that the foal she was looking at was her. Or rather, a foal version of her. It shook as it walked, not with unsteadiness, but instead rippling, as if made of...
Twilight's thought process trailed off as she remembered what had happened. Another foal slid out, followed by at least a dozen more. The first two began suckling on Twilight's teats, the milk practically exploding out of them. Each foal that popped up was the spitting image of their "mother", save for the downsized appearance. After thirty or so minutes of foals switching off between teats and more of them sliding out of her slit, her abdomen and teats were back to the size they had been before she had fallen into this cave.
Suddenly, the foal trotted to the other corner of the pond, each lapping up goo. Slowly, the goo pony she had seen before, still translucent and blue, emerged from the goo pond again, ready to begin the process once more. Twilight happily accepted, enjoying every last bit of it.
Author's Note
I know, the same thought was racing through my mind the whole time too.
"Why did I write this?"
But seeing as I'm fucked up in the head, I figured why not. 
Don't ask.
It's safer that way.
