A habitual bolter or a habitual rearer: which is scarier? It's a tie as far as I'm concerned. But there's something worse: flippers. Sometimes horses rear, lose their balance (especially if the rider pulls on their face), and fall over backwards. That's not flipping. A true flipper does what this horse does (BTW, pay no attention to the "Man dies!" description. It's perfectly obvious no one is even hurt.):
I'd never actually seen a true flipper in action. You can see from body language that both the groom and the jockey have a pretty good idea of what's coming, although it's interesting that the horse never really lays his ears back. Is it possible to rehab a horse that does something like that?