It's weird, we'll be out happily hacking in the field, and then suddenly, a bush will move, or a helicopter will fly overhead, or a squirrel will come down a tree, and he jumps a foot and runs off. It doesn't matter what we're doing too, we could be walking on the buckle or schooling cross country, but he always find something to spook at. He's been like this since the moment I got on him, and I've just recently tried using Parelli methods to calm him down. He spooks in the arenas too, but once he spooks out in the polo field (where we hack at my barn), he's almost unstoppable. He seems to do it less when I lounge him before riding, but I did lounge him before he threw me off.