As for horse droppings - it's just another trail obstacle to improve your bike handling. One is the amount of litter. It is obviously bikers. I seem to notice more GU packets and assorted power bar wrappers on the ground. Now I know it's possible to actually drop one and not know it, but judging from the amount of them I see at other trails, it's not an accident.
Somebody is just plain lazy.
If you brought it, you sure as hell can take it home with you. I'm not sure what kind of agreement was made in getting the trials open for bikers, but it should not have included sharing the trails with horses. Horses are the equivalent of motorcycles to trials. They tear it up, badly. I used to live in an area where the bike trials were close to a boarding stable.