Bit of an unusual commission for A-non-mouse.
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The process would be strange enough, even if something wasn't clearly going wrong.

Radian had never really been much of an urban explorer, so he didn't know why the urge to enter the lonely building had been so strong. He'd certainly never seen the building before, now that he thought about it. Honestly, in hindsight it couldn't have been more suspicious. Though, to be fair, no reasonable person would have expected... this.

The entry of the building, barely impeded by a broken door hanging from one hinge, had been drab and dusty. Covered in leaf litter blown in from outside and inexpert spay paint. The run-down look had contrasted sharply against the sight of a surprisingly intact convener belt, beginning in the middle of the room and disappearing into a brightly colored door frame. He had tried to shine his flashlight into the dim blackness beyond, but couldn't make out anything. So, he'd done the most logical thing to do in a definitely-not-haunted building and climbed up onto the conveyor belt to move farther in.

It had been a mistake.

As soon as Radian stood upright, he found herself unable to move. Oh, he could shuffle from side to side, and even turn a bit, but he found that he couldn't make himself walk, or jump back off the platform. He gulped nervously, starting to sweat as he heard a grinding noise. Unexpectedly, the surface beneath his feet lurched, and he dropped his flashlight in surprise. And then he was moving.
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The belt carried Radian into the dark door frame, and he strained his eyes trying to see anything in the dark. Only to be left blinking when the world around him suddenly lit up with a bright, cheerful glow.

He was standing in a long, blue hallway. A line of mirrors covered one side, and he stared at his nervous looking reflection as the belt carried him slowly along. Somewhere, a soft, happy tune started playing through a disused set of speakers. Slow and scratchy at first, but picking up speed and becoming clearer as the friendly music got going. And then things got even weirder.

Suddenly, a section of the wall in front of him opened up. Out of it emerged... well. Several mechanical arms and a hula-hoop. He wasn't even sure what she was looking at. At the base of one of the arms, he could just make out a label that proudly proclaimed the machine the "Fluter-ifier." Whatever that was. He was startled by a hissing sound and then a series of holes on the inside of the ring began to produce beads of pink and yellow liquid. The strange, shiny substance seemed to defy gravity, flowing out and filling in the inside of the hoop, until it looked more like a disk. A big, stretchy, goopy yellow obstacle, right in Radian’s way.
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He tried to awkwardly step around the thing, but was caught off guard when it swooped under him instead. His shoe landed in the brightly colored stuff, and he found herself unable to free it. He was about to try and pull his foot out of it, when something shoved him from behind. Flailing wildly, he just caught sight of another metal arm before the other foot landed in the hoop and it started to rise.
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Radian quickly found himself unable to pull his feet out as it rose past his ankles. As it did so, the areas it passed were left feeling... strange. He found himself unintentionally balancing on tip toe, rubber squeaking between him and the floor. But... he could no longer feel his shoes under the strange, shiny new layer. Or his socks. Or... his toes?
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The ring moved up, gripping at his legs as it covered them. The sensation was indescribable. It felt like it was gripping him so tight it was melding with his skin. Like it would never come off, no matter how much he pulled. Radian gasped, reaching a hand down to try and slow the ring as it climbed upwards, but he couldn't stop it. It pulled up over his hips, wrapping him with with a squeezing, squeaking pressure, and just kept going. And as it passed, the melding, pressing feeling did… something. Looking down as he fought it, he realized that his kicking feet no longer looked like feet at all. Two long, blunt cylinders of buttercup yellow rubber paddled below him, and a swishing tail of long pink hair swayed behind them.
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He cried out in surprise as the ring passed over his chest, forcing his arms up. Under it, he could feel several strange, unexplainable things happening. On his back, two lumps began to press outwards, somehow feeling like they were both pressing tightly against him and pulling outwards with tremendous force. With a sudden, bouncing pop, they burst away from his torso, fanning out into small wings. And, on the opposite side of his body, two... other protrusions began to swell. He blushed, furiously, as they grew, though these felt more like he imagined a water balloon being filled would.
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His… or her? Her wrists were grabbed by more of the robotic arms, and shoved down through the filmy layer. He was afraid for a moment that they were going to meld into blocky, useless hoof-things, like his feet had, and was relieved when they instead resolved into petite fingers. And then it was moving up his… up her… neck.
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He automatically tried to yell out, only to get a mouthful of strongly tasting rubber as it covered his mouth and nose. Radian panicked for a moment, as he raised his new hands to scrabble at his now covered jaw, and closed his eyes as the ring passed up over the top of his head. For a moment, he was left encased in a rubbery prison and his heart thundered in her chest as she pulled at it. And then, the squeezing, melding pressure began to shape her face.
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She gasped, mouth popping open, as her jaw expanded out into a short muzzle. Her ears buzzed, feeling strangely placed on her head. And she felt something else. Something... deeper. A different sort of pressure, like a second, tight, rubbery coating was being stretched over her mind.
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Fluttershy pressed in around her like a rubber glove. She was still herself, but the restrictive borders of the external identity would only let her stretch so far before snapping back into place. Was it shaping her, slowly warping her the way the rubber had changed the body under it? If she stayed like this, would she fit it perfectly after a while?
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With a gasp, Radian-Shy's eyes popped open. She looked down, speechless, at the smooth expanse of gleaming yellow rubber. The mirror on the wall showed a soft, shiny anthro pony staring shyly back at her.

And that's probably where it should have ended.
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But the conveyor belt was still moving forward.
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And the grinding sound coming from the walls was getting louder.
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The hoop seemed to freeze, stuck, halfway back into its compartment. Its arms twitched and sparked, and the music overhead glitched and skipped, scratching like a broken record. Radian… Fluttershy… Radian flinched away from the noise, wings fanning instinctively, but she was still trapped on the conveyor belt.

Another section of the wall cracked open, and lurched out to block the conveyor belt’s path.
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Large, blocky slabs of metal started to close in around her. She could just see the word Pinkie-atron printed on the side. She flinched and shrieked out a high pitched cry, Fluttershy’s timid mind squeezing around Radian’s in fear. She managed to keep her arms tucked in, away from the hand-shaped molds grabbing at them, but was unable to evade the two clamping halves of the one that snapped shut around her legs. In her distraction, one of the clamps caught her hand. She shrieked again, and then shuddered as another, even more indescribable sensation began.

The two halves of the mold heated up. Pleasantly warm, and then searing hot. But somehow, it didn’t hurt. It just started to… well. To melt her. She felt a warm liquid flowing into the molds from somewhere else, and she wondered if it would have encased her the way the ring had, if she had gone through it first. But with her body already made of yellow rubber, all the new liquid was doing was mixing in with it.
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And overflowing.
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With a bubbling hiss, goo began to overflow the top of the mold. It didn’t flow away or escape, seeming instead to meld with the rest of her body. The tips of her fingers popped out through the edges of the mold on her arm, like balloons being squeezed to hard. Her hips and belly began to bulge around the lip of the mold. Beneath, she could feel any trace of… of the old body she’d once had, melting and swirling away.

The larger mold that should have held her head and torso sparked and ground its gears, not moving any further. But it didn’t really need to. The excess goo flowed through her, pumped by the other molds, and swirled up across her head and chest. Her face flushed bright pink. Both from the latex that flowed up into her hair and from the sudden feeling of more bouncy liquid mass being added to her chest. The liquid rushing through her head began to press at the edges of Fluttershy, inflating her like an overfilled balloon, and Radian felt a sudden pang at the thought of losing her.
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Suddenly, with a hiss of steam, the lower mold popped open. The cold air on her flank was a shock, sending pins and needles prickling down her sides. And then, with a loud BOMPH, she solidified.
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Her wide thighs bounced into place like a balloon, squeaking as they settled. Her glossy breasts perked up, popping out into round spheres, like air-filled beach balls. The smaller mold let go of her now pink arm and she rebounded away from it like a rubber band.
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Her head felt like it was filled with helium. Fluttershy was stretched, and stretched, and stretched, and finally, with a snap… came back into place around Radian’s mind.
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Only now she was sharing the space with Pinkie.
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She… They? They giggled slightly. Absent and ditzy sounding, and a bit nervous. Radian felt the edges of their thoughts that were shaped by Fluttershy, and the floaty, bouncy, lighter-than-air buzz that was Pinkie. It was hard to tell who was what and what was who. And who was even in charge of the body anymore, anyway? Was that Pinkie’s leg that the purple goop was climbing up? Or Radian’s?

… Wait. … What?
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Pinkie-Radi-Shy looked down at the puddle of purple goop that was leaking out of a hole in the broken wall.
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There was a face in it.
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“AHK! Hey no fair! There’s not even enough room for the three of us in here!” came the thoughts bubbling up out of Pinkie’s floaty cloud mind. “I- I sh- should make room. I d-don’t want anybody to feel left out” came the stuttering worry of Fluttershy. Radian was just trying to fight off the purple gloop.
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It didn’t do any good. The flowing liquid latched on wherever it could and started forming a new pony body around its host. Except the host already had a pony body. A new arm formed. A wave of dark blue ran down the tail, trailing stripes as it went. The chaotic scramble continued for a minute and the conveyor belt moved ever onward.
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It passed several more malfunctioning stations. A cluster of bouncy white spheres rolled up onto the belt, but there were only enough of them to cover one leg. A single drop of blue goop, dripping from the ceiling, happened to land on one yellow wing. And the grinning face of the purple goo struggled valiantly to pull itself over the head of its new host. It was driven by one thing and one thing only. Its single, all-consuming drive.
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To make a new friend.
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The face of Twilight finally jammed itself over the struggling pony’s face. Or, well... mostly. It decided it was good enough and fused with the underlying substance before it could be pulled back off. For the second time that day, Radian gasped and opened new eyes. Except… there was no mouth to gasp with. Only a painted rubber grin, smooth as the rest of their body. And the eyes… there were three of them.
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Twilight sloshed in their head, liquid churning with the puff that was Pinkie, and the shell that was Fluttershy, and the very confused bit that was Radian. And in that sloshing, they all started to mix together a bit better.
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Finally, finally, something in the conveyor belt gave out. They stumbled as they were released from its grip, and pinwheeled their three arms to avoid falling on their face. Not that it would have hurt all that much, seeing as they didn’t think they had a bone in their body anymore. Three eyes, blinking somewhat out of sync, looked up at the mirror at the strange, rubbery creature in front of them, and tried to figure out just who, exactly, they were now.