Mary found her eyes drawn repeatedly to the mythical section of the box, and figured she might as well try something truly fantastic for her next vial.
"...but I don't know if I'd lose my human mind if I became a dragon or a unicorn or something... maybe I should try the mermaid vial? But will that turn me back to female? And I don't have a pool in here - would I dry out before I can fill the bathtub?" she contemplated her options, finally settling on the vial with an image of a centaur. "Perfect!" she thought, "I can remain half-human while experiencing a really different transformation! Bottoms up!" Downing the contents in one gulp, she backed into the center of her living room, giving herself space for what she expected to be rather large changes.
A tingling flush spread outward from her chest, goosebumps raising on her muscular torso and flowing down her arms and legs, even up to the tips of her ears. Her new cock pulsed, stiffening once more as her balls throbbed, the tingling racing through them and further back, up between her buttcheeks and over her anus. "Oh geeze." Mary grunted, feeling a pleasurable pressure building at the base of her spine. "Am I growing a tail first?" She twisted to look over her shoulder, getting a glimpse of her firm, masculine buttocks as they seemed to plump up, pushing out roundly and then rapidly distending to inhuman proportions as the beginnings of her equine body formed. It was pushing backward, and Mary struggled to maintain her balance on two feet - her spine was lengthening, new vertebrae popping into existence as her rear end continued to grow.
The pleasure remained, however, building as she felt her new equipment shifting, sliding back with the expansion of her lower torso. Reaching her hand between her legs, her rough palm met smooth skin covered in a fine fuzz, fingers sliding back, back, back until she couldn't reach any further. Her rear end suddenly felt much heavier and she was forced to brace both hands on the back of the couch to keep from tipping backwards. Twisting her head, she saw two protrusions pushing out from her new hindquarters, growing downward as hips, haunches and hind legs formed, quickly reaching the carpet and helping to support her new bulk. A third protrusion formed as her spine continued to lengthen, her tail finally arriving as a fleshy limb, quickly covered in rapidly growing hair to match that on her head. Muscle piled on, thickening her new hind legs until it truly looked as though the back end of a horse had attached itself to Mary's rear end!
The tingling was back, centering in her front legs this time, and Mary looked downward to watch in fascination as she was pushed up onto her tip-toes, her heels stretching back as her shins shortened and more muscle built in her already muscular thighs. Keratin grew, encasing her toes and leaving her with hooves at the bottom of her new fore-legs. "This is so awesome!" Mary whispered, lightly stomping one hoof on the plush carpet. She could feel her new extremities, her tail swishing behind her without conscious thought. Her black body hair had thickened, covering her lower half in a dense fur that lessened only slightly to more human hair over her torso. She was definitely hairier than she'd been as a human male though! Thick curls covered her pectorals and she could barely see the skin on her forearms any longer. She felt something tickling her neck and realized that the hair on her head had grown out again, long, wavy strands covering the fact that it grew out from the back of her neck as well, forming a mane that stopped somewhere just above her shoulder blades.
Oddly, she felt her ears twitch, and she reached up a hand to find soft, fur-covered cartilage sticking upwards out of her hair. "Horse ears, too?" Mary wondered - she hadn't thought the changes would affect her above her human waist...