My OTTB isn’t phased by many of the things that I would expect him to spook at (weed whackers, atvs, flapping caution tape, dogs running out and barking at him, deer jumping out of the brush on trails, etc). But we were in the warm-up arena at a show one time and a rider fell off her horse — he spooked SO BAD!
Oh, and hills. He’s gotten better with practice, but the first time I took him up and down hills he acted like there was a cougar suddenly attacking him from the trees or something!
Also, my friend has a horse that has the strangest affinity to donkeys. He’s never lived with or been around donkeys in his life. Kind of the opposite of a spook, he bolts uncontrollably TOWARD any donkey he sees.