my heart sank as i read of your pristine conditions ruined. here in the east, some state parks have clearly marked bridle trails (as well as x-cntry ski trails). however, the owners/riders often don't like to stay on them and off the easier backpacking trails, so one needs to be careful where one steps. i stay off of the bridle trails. they should have a similar courtesy - too bad, many don't.
grew up right across the street from a farm that also raised Morgan Quarter horses, but don't really know much 'bout horses myself. wonder why someone would take a questionable horse (one easily spooked) on the trail? could be dangerous for the rider as well. in dogs such behavior (excessive fear) generally points to either poor genetics (too much inbreeding before out crossing with another line) and/or poor socialization (i.e., exposure to the intended situation or a similar situation) as a pup.
i know this much, ever since i was a kid, the few time i've actually encountered a horse on the backpacking trails (instead of just seeing evidence of them), i don't move to the low side. it's considered potentially too dangerous. i just move a bit farther off (if possible) the high side of the trail. don't see why i should be the one placed in the greater danger on the "no horses allowed" backpacking trails.
fortunately, out here, many trails have a lot of higher, steep, rocky places that horses can't get up. in many places, i can just move a bit higher, above the "*BEEP*"-line to get water from a small stream.