Katie decides to take the rest of the female vial, just to see what will happen. Worst case scenario, she needed to experiment more with the remaining vials. Katie held the vial to her muzzle, and in one gulp drank the other half of the liquid. She instantly felt the effects.
Her breasts grew even larger, until they were probably H cups. Katie cupped one with her hand, and guessed it weighed at least ten pounds. She saw something white dribbling out of her nipple. Milk. A wave of pleasure rushed through her body, causing more milk to leak for her enormous tits.
Katie saw her cock slide out of her sheath. But it wasn't as long as it had been before. She almost jumped for joy as she saw her male endowments were shrinking away. Though it remained erect throughout the change, her cock shrunk away until it was only an inch long. And her balls were sucked up into her body, in their wake leaving a tight slit.
But then Katie saw her horse-pussy moving backwards, engulfing the slit. What had been her penis became her clit, a large swollen button that was black just like the rest of her pussy. Her marehood was dripping, with each beat of her heart Katie saw her lips opening and closing. The hot, wet flesh inside smelling so animal. The scent told Katie she was in heat. And she needed a stallion's rod. But she didn't have one anymore.
In desperate need of a release, Katie once again inserted her own hand into her twat. It wasn't enough. No matter how much of her hand went inside, her pussy demanded more. Katie looked up at the vials. Could she, SHOULD she, change herself again? The female vial had given her back her "lady parts", though she was still obviously half horse, and thrust her into this tormenting state of lust. The only out she saw was growing a cock again and fucking herself.