You say that you would never rape any animal, but consent isn't just about whether or not someone objects, it's about someone verifying that they want the sexual attention they are recieving. No animal can give consent. You can never know that the horse is letting you fuck her because she WANTS you to fuck her or because you're her master and she's obeying you. It's the same reason you can't fuck a five year old (besides the obvious physical difficulties), the same reason you can't fuck a retarded person, the same reason you can't fuck a bitch who's out of her mind drunk/high, and the same reason that any slave master fucking their slave was automatically rape.
Unless your name is Eliza Thornberry, you can't call anything you do to an animal "consentual" activity. So stop it, horse fucker!
Now, you say that you've never actually acted on this desire. That's good, and my advice is to keep it that way. Nobody can be blamed nor hated for the things they think, feel, and desire. Buy a horse calendar and fap your balls off year-round. Enjoy your thoughts and feelings in the privacy of your own mind, where those thoughts and feelings that could harm others (even animals) should STAY.
All My Hate,