My worst "garbage dumps" have actually been those left by *ATV* campers, in my experience they're the worst offenders when they're allowed into an area. The horse packers here don't tend to compare, although they can be bad (cans left around, etc.)

But let's not confuse the horse with the idiot sitting on it. Most of the time I have a real problem with sharing the world with ATV's and horses it has a lot less to do with the ATV or Horse per say as opposed to the ignorant clod sitting on top of it. but hey, my worst on trail experience was with a mountain biker. People are still the biggest problem. All of the above (ATV, horses, bikes) can cause very little damage and be very tolerable when used responsibly. When they aren't they're awful..