"I wish I was a cow!" you say into the machine, then you get a shock when your clothes vanish and you suddenly find yourself standing naked in the alley behind the magic store. You put your hands over your privates to cover yourself and look around in both directions.
"What the heck?" you try to say, but a loud bovine moan comes out of your mouth.
You feel a swelling feeling in your lower belly and look down. It's at that moment that you realize your chest is now flat and featureless but your abdomen... isn't. There's a bulge swelling up, turning light pink, and starting to sag down between your legs. There's some little bumps starting to form on it. Actually, they're not that little anymore... whoa! they're... they're...
You reach your hands down and start grabbing the long protrusions, confirming that they are in fact your nipples, that they are no longer on your chest, and that there are four of them. You run your hands all over the great big udder you never thought you'd have down there. You look back at the machine. It actually works?
Your hands grow out, your upper arm shrinks, some of your fingers fuse together, so that after a few moments you just have forelegs. What's left of your fingers gets overtaken by keratin and you find yourself starting at your new pair of hooves.
You feel a powerful surge out of your bottom, causing you to moo loudly, and when it stops you feel a new weight hanging down from your back. It doesn't take long to notice the long tail swaying between your legs. You can't pick it up in your hands, but you can swing it all over the place and slap it against yourself at will. It also doesn't take long to notice the long black hairs growing out out from the end.
Your ankles hurt so you lift yourself up on tip-toes. Of course, you can see that your toes are turning into hooves, too, and before long you start wobbling around on your back legs unable to stay upright. With another moo, you topple over and let your front hooves take your weight for the first time. That's better. Your face pushes out into a long broad muzzle, and your ears push out giving you a pair of round floppy appendages that swing in whichever direction you choose to listen. A fuzzy feeling covers you and you look back to see patchy black and white fur materializing on yourself. After a bit more bulging and thinning in various places, and quite a bit of beefing out, it's all over. You're now a farm animal standing around in a back alley.