In my opinion some horses are just built like that. I had one who would spook at fairies, in other words nothing!!
We would be riding past a spot on the arena that we'd been past a million times the same day and it would suddenly become the scariest spot in the world. At shows he would spook at grass spots that were different shade then the rest
It sounds like he'd do the same thing as your horse, the 4 feet jump stop or jump sideways, I would always just try to ride him forward, I'd never berate him as to him he was honestly scared. If he did a poop on the circle and then we came round again, he'd even be scared of his own poop!!
This horse taught me a lot about becoming supple and going with the horse, cause if I wasn't supple then I'm pretty sure I would have been on the ground!
This was when he was a youngster, he's 8 now and though he's not as bad, he still finds fairies.
He sounds like a horse that needs the rider to be confident and perhaps the last owner was, so didn't have any trouble.
Be patient with your horse, don't tell them off when they spook you'll just scare them more and eventually you'll find that they will trust you more cause they will feel that you are not scared so that will give them more confidence.
Sorry about the huge spiel, good luck!