I have the same problem with my OTTB gelding. He 's fine with all horses until he gets under saddle, especially in a group of horses, when he pins and makes to bite, rounds his back and gets behind the bit and fools around. I'm feeling that is that it is coming from experiences at the track, even though that was seven years ago. He also has to be the first horse on a trail ride. Sometimes it's OK with one horse, especially in the arena, but with a group he's always uncomfortable. Luckily, I usually prefer to ride on the trail by myself! When he acts up with other horses, the only thing I can think to do is press him forward, which usually helps. Interesting to hear others with the same experience!