I second the check over by a vet, chiro, etc … my 3 year-old had a similar problem when tracking to the right on the longe … he also has scarring on his right hind ankle and hoof (purely cosmetic) from putting his hoof through a metal wall of a barn before I got him … he would melt down at anything other than a walk while tracking to the right on the longe, whether I had a saddle on his back, or nothing but halter and boots. A trainer I brought in to check things out and see what he could do is who suggested a chiro for my boy. One session with the chiro, and he was moving so much better to the right. No problems since then either. He has a 3rd session coming up soon, and my mare has her first session coming up, as she is showing slight difficulties tracking right at the canter (she has been a broodmare for the last few years) … the longe may be boring, but it does give us opportunities to spot things that are wrong with our horses that may not be obvious in hand or mounted. It also gives me a chance to see what their frame of mind is. Longing is also good for conditioning a horse a bit at a gait before making them cart our butts around at that gait too … sure, they can run, buck, kick, and play just fine, but that is in their natural state without tack and a rider.