Ya know, what's the worse that could happen? Go up and ask if you can pet the horse, they may think you're a little odd, but explain that maybe you used to live near people who didn't like people to interact with their animals or whatever. Make up a little something if you have to. Try to catch them when they're out with the horse maybe too, then it wouldn't be very alkward you know?

It does kind of sound like OCD, maybe it isn't but it really sounds like it would help you out to talk to a professional about it. There are medications and therapys that can help you out a lot!

Try going up to the horse as close as you can stand it, try petting it and doing everything you can to over time make you more comfortable with the situation. It's what the doc you'd see will probably have you do, get a head start. ^_~

I can tell ya though, I don't think anyone would find petting a horse a malicious act that warrants legal action at all.

Good luck! Do keep us posted, and I hope everything can turn out for you. ^_^