I’m not white knighting for them.
I’m saying the OP’s complaints are totally useless if the horse people are on the [narrow] road in front of the house.
Right or wrong ethically or morally doesn’t matter when the horse people don’t give a flying F. It’s the law that matters at that point, and whether or not it’s worthwhile to continue to escalate things.
I can’t imagine the horse owners saying “we are entitled to poop on your driveway.” That’s setting my BS meter off. They are entitled to ride down the side of the road, where the OP does not own it or it’s a public easement.
They ARE rude by leaving the poop there, but it’s not against the law in any country jurisdiction that I know of. So the OP continuing to escalate things is a total waste of time.
It would have been more beneficial to the OP to remain friendly here than do what’s been done. Because the end result is the same - the poop is still there.