I have a 4 year old paint gelding, he's about 16.2hh and really sweet. He's not fully broke, but he's a nice ride. But I noticed he bobs side to side and the faster he goes in a walk the worse it gets. But when he's trotting it stops and he holds it high. I got him this past may and he's never bobed then, it started when we got my 3 year old Paint, h edose it in the saddle and on the ground, when he's hyper and wanting to go back to the pasture and the other 2 hroses he has more of a bob.
What could cause it?