You're lucky to live near relatively untrodden wilderness. In my experience as a hiker, many trails in the Sierra Nevada have been beaten into dusty trenches by horses, mostly thanks to pack companies. Quite a few in the southern Rockies have clay-rich soil, which horses will destroy when it is wet (so will bikes, but mud-riding is awful, so most riders will stay away). I have also been in some areas where horse traffic is the only reason that unmaintained Forest Service trails have not grown over.

I think banning commercial packers from the Wilderness would go a long way to fixing the problems with horses. Perhaps with some sort of licensing system, mountain bikers could be allowed as well.