Both my horses grew up with donkeys so mules don't bother them. If Thor sees a cow though watch out cause he's going to run for quite a ways to get away from that horse eating monster. It's so embarrassing having a 1600+ lb horse scared to death of a calf.
I have had others board their horses (who had never seen a donkey) with my donkey Eyore before and most of the time they just give him a good looking over then treat him the same as another horse. I find that the horses seem calmer with the donkeys around.
I don't trail ride right now but I do take long walks with my boys. Baldur is in trail horse training right now. In September he comes home and I get to finally hit the trails again after ~10 yrs.