You step out into a small corral, blinking in the warm sun. Several things hit you at once. First of all, you feel powerful and massive, and are standing on four legs. Second, there are two people watching you. And third, the other horse is a Clydesdale mare in heat.
A quick look back explains all of that - you are a massive Clydesdale stallion, with markings similar to the lovely and fragrant female. And as a stallion, you recognize her need and are responding to it. The people watching must be your owners. You are in a breeding pen, and unless you fight against powerful instincts and desires of this new body, you are about to mount the mare!
So what's wrong with that? You're a stallion and she's a mare - but you aren't really a stallion, are you? Is this some trick of the mirrors? Having sex with the horse might lock you into being a horse yourself! But then, maybe you are already stuck here. Or mating might be what allows you to go back. What should you do?