a2, there are many studies that show mountain bikes, horses and hikers have very similar impact on a trail. Maelstrom makes a good point, A well designed trail will make all the difference. I'm sure water will kill a bad trail faster then mtn bikers and horses combined.

I have never had a conflict with trail horses when mtn biking. I always stop and dismount when passing a horse head on. When passing a horse from the rear, always be in vocal contact with the horse, and don't make any unpredictable moves.

I have however had a few conflicts when riding on a multi-use trail. There is a clearly marked horse trail that runs parallel to the bike path, but is rarely gets used! When a horse craps on the bike path, you have to wait weeks for rain to clean it off. This bike path is always covered with crap. I really wish the horses would stay on there part of the trail, or if the riders would come out once a week and clean up the crap on my side of the trail.