Horses leave round holes. They fill with water and in a short time, the sides collapse. Before long, the tracks disappear. They might be bumpy for a while, but nothing washes out. It's no different than elk tracks on steep side hills that don't cause erosion.
A bike leaves a long continuous groove that causes major erosion. There's no comparison.
In the late 60s and early 70s I spent 6 summers on FS trail crews in Idaho and Alaska, respectively. RC has it right on wheeled vehicle damage to trails when wet. Horse use is much less so, but still pretty bad.
As to not being "designed for such use - blame the FS" - well that's mostly ignorance talking , tho actually true in part. Many of these trails were never "designed" at all - just established by use between places people wanted to go. Quite a lot of them over 100 years ago, when no one used wheeled vehicles unless they built a road, or improved the route in well used questionable areas.
