I give them as wide a birth as possible but in my opinion, if the horse is out where it will encounter motorcycles or ATVs etc....the rider should have control of the animal. It's really not my responsibility to make sure their horse doesn't get spooked. I don't buzz by them at high speed but I'm not going to sit and wait while they pass or wait for instructions as to how they would like me to pass. I treat them like anyone else I meet, give them room and slow down as I pass....
"Beer cans explode with a nice loud pop when they are hit by a Heidenau K60 at 40 miles an hour. Now my bike smells like last call..." Me