We have horse trails at our local city lake. The only rules they have are horse camping only allowed in designated areas. No riding through campsites, no horses on the swimming beach, no picketing to trees and you have to take your poop out of the campsite when you leave. Most places you have to remove the poop and just put it in a none human area like spreading around trees or underbrush. I would start with the Department of Natural Resources for your state.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France