My young horse bolted apparently for no reason once. It wasn't a spook, same saddle, riding in a halter. We were just walking along and he bolted, even after I fell off he continued to run the ring for a dozen laps or so before finally settling down and eating. It hasn't happened since and I'm not worried about it happening again. I think he got overwhelmed all of a sudden. I hadn't done any groundwork beforehand and he'd only had a few rides on him. But he appeared calm so I didn't feel the need to longe him.
I don't worry about it happening again because there were some things I did wrong that day, I should have checked him out a little before mounting and I did ignore the fact that he walked off while I mounted. He also seemed a bit agitated in the saddle. I'm just a bit more careful. I've never come across a horse that is just mental but I don't doubt there may be a few out there...
