Before you can even respond or rethink your decision, she blows some fairy powder in your face and waves the wand, you feel yourself start becoming heavier and you look to your hands, seeing your fingers merge together and turn black, becoming hooves. "What are you doing to MOOOO?!" you try to speak as she laughs and rubs your head. She pinches your lips and gently pulls, making your face elongate into a cow's muzzle. She tugs at your ears, which surprising seems to turn you on, and they become larger and floppy. Your body starts growing fur everywhere as your clothes start ripping.
She looks down and sees your cock, "Aww is this exciting for you?" she teased as she reaches down and strokes your cock, "MOOO!" you respond, mooing in pleasure. "You are going to be my COW familiar, not a bull sweetie, so we will need to get rid of this" she says as she winks at you, and you feel your cock shrink and shrink into nothing but a slit that she teases, making you even more turned on.
You feel pressure building up as you fall to all fours, and start growing an udder, it fills with milk as your changes finalize, and all you can do is look up at the much taller witch now, she claps her hands excited with her work, "You make such an excellent cow dear!" she laughs as she holds a mirror up, and all you see is a common cow staring back at you.