No creative title because reasons. :P
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Amber awoke to find herself in a grey containment chamber. The short Asian girl looked around the room, finding it devoid of life. She pressed a hand to the one of the mirrored walls, a frown of concentration on her face.
No sooner did her hand meet the cool glass than a surge of pain ran along her right arm. Amber groaned with pain, pulling up her sleeve to reveal that there was a small dot in her arm, as though she’d been injected with something.
She didn’t have much time to reflect on this, another surge of pain running through her arm. She clenched at her bicep with her left hand, gritting her teeth as her hand started to shudder and twitch uncontrollably.
Amber’s eyes widened as the muscles in her arm began to expand, her fingers cracking as they ballooned out into what almost looked like an animal’s paw. A yowl of pain let itself loose from her throat as her thumb retracted into her wrist. Her shout rose in pitch as sharp claws burst through her fingertips, merging with her nails.
“W-what the fuck?” Amber gasped, panting with exertion. She glared accusingly at the injection site, a small gasp coming from her throat as she realized her skin was paling around it. She could only watched as her arm became a light shade of white, black spots appearing across her skin.
Her arm itched madly as her hairs stood on end. Amber watched as countless hairs forced their way from her flesh, forming a thick, fluffy coat. It had the same pattern as her skin, black spots scattered throughout the carpet of white.
“Wh-what’s going on?” She looked at the mirrors surrounding her, her left hand nervously clutching the paw that had become her right. Her ears twitched as she continued to beg for mercy, creeping up the girl’s scalp without her knowing. They drew to feline points, white and black fur covering them, too.
The serum had now taken hold of her body, and there was no going back. Amber ceased her protests as the sound of tearing filled the room. She looked down, yelping as she saw her right foot had burst through her thin sneaker, expanding into a large animal’s paw. Her left foot exploded through her other shoe a moment later, and Amber was forced to all fours as her legs gave way beneath her.
She screamed in pain—not from her changing bones, but from her suddenly-tight jeans. Her scream deepened in pitch until it was a bestial roar, drowning out the sound of the seams of her pants giving way. Her bones continued to crack, raw muscle toning her legs, fur covering them.
Thinking quickly, Amber rolled onto her back, fumbling at her belt with her good hand. A few moments of this and her waistline started to ache. She whimpered, quickly slashing through the leather with her animal arm. Just in time, too-her waistline surged outward, the uncontrollable changes reaching her torso.
Amber roared again, her hind paws scraping at the ground as she arched her back. Her spine cracked, the joints popping in rapid succession. Amber rolled onto her stomach, ragged breaths escaping her slack jaw as her body was filled with an uncontrollable heat, her rapidly growing fur coat proving too thick for her current environment. Her ribs cracked as they expanded, her torso nearly doubling in size.
Amber’s tongue lengthened as the fur reached her neck, her repeated gasps for cold air washing over the longer appendage, even as her teeth expanded into impressive fangs. Amber looked back to the mirror, seeing her animalistic face, her savage jaws. She whimpered as it pushed out into a muzzle, the dull cracks assailing her ears. Her nose flattened out, becoming a dark grey. She sniffed at the air, suddenly aware that she could smell both her human self, and the feline predator she was becoming.
Amber worked her jaw from side to side experimentally, seeing her reflection mirror the action, whiskers pushing their way from the animal girl’s cheeks. Her eyes became a hue of gold as vivid colors disappeared from her visible spectrum—not that that mattered to her. All she was focused on was the black and white cat in front of the mirror.
Her rear stuck prominently in the air as a large tail burst from her bottom, fluffing out immediately. The fur there seemed to be thicker than the fur on the rest of her body, and she slapped it against the ground as she looked to the mirror one last time.
Amber stretched out her human arm, pleading to make the changes stop—no, to undo them. It was no use, and she could only watch as her hand shifted to a paw, her arm transforming until it was a mirror of her other. Amber the girl shook her head, squeezing her eyes shut as her skull shrank, her brain reworking. When she opened them, she was Amber the snow leopard, animal instincts overriding human thoughts.
The wall slid open behind her, and she spun to face it, baring her fangs as she growled. To her surprise, a wintery expanse greeted her, and she slowly padded outside, eager to hunt. She could still remember her time as a human, still think as one—but she had no need for those thoughts right now, she needed the predator.

“Mission accomplished.” A scientist grinned from behind the one-way mirror. Raucous applause greeted his statement, the gathered researchers watching the snow leopard make its way into the enclosure.