Katie stopped in front of the large display of vials. Some colored, others full of clear liquid. She paused when she noticed one that had the image of a female on it. Thinking it would return her to her normal body, she popped the cork off and drank half the vial.
She set it down on the shelf, and waited for the changes to start. Katie took off her clothing, expecting herself to shrink so much that nothing would fit. Instead, she found herself growing. Her chest, more specifically her pecs were turning into breasts. But not human breasts. Her nipples were far too puffy and dark, and her chest was still covered in horse hair.
Katie's chest ballooned until she had breasts the size of basketballs. She felt her face reshaping itself, losing the hard edge and becoming more feminine. Katie was relieved to feel something pulling at her crotch. She assumed it meant she was getting her female genitals back. When she looked down she came face to face with her erect cock. It wasn't shrinking at all.
"What the hell is...." Katie was cut off as her cock's head popped into her slightly extended muzzle. Her enlarged tongue probed the tiny pink opening, tasting the pre and longing for more. Katie couldn't stop herself. She started sucking as much of the tip as she could.
Her balls felt different, and as she continued to suck herself, one hand went down to lift her balls. That's when she found her vagina had indeed returned. Or A vagina, because the womanhood she sported was NOT her own. It belonged to a mare. The giggling mass of flesh stretched from the back of her balls to the edge of her puffy horse asshole. And it was so large. Katie had no trouble sliding her hand inside.
She came as her new pussy engulfed her entire hand. Thick chunks of horse cum hit the roof of her mouth, and then quickly washed down her throat. Katie removed her hand with a loud, wet smacking sound. She sat down on the floor, her breathing ragged from the intense orgasm and the transformation.
She looked at herself, realizing that the vial had only returned some of her female attributes, and she was still very much part horse. But there were still dozens of vials sitting on the shelves above her long ears. She just needed to pick the right one.