He is terrified of squirrels. And most small things, from chipmunks, to a nutria we saw one day, my Cavalier King Charles mix, cats, etc. But oddly, blowing bits of paper and other scary things don't bother him. I discovered this while leading him the long way back to his paddock one day, because he's fun to walk with and lets me hop on to ride bareback. We were going down a path that let in between trees and a squirrel popped out from the ditch on our left, then ran across. You would have thought the thing was an axe murderer! I had to let go of the lead, there was no holding onto him, he's all muscle. He takes off, runs out of sight (towards the thing, he's a genius) and I take off after him. Eventually I find him attached to my neighbor's mailbox. Thankfully I had not taken him out of his bitless bridle, and the reigns were caught around the pole (I may hate the bridle, but the thing is strong). He had about ripped the thing out of the ground! I finally got him into his pasture, where he promptly hid behind my Walker gelding for the rest of the afternoon. A friend of mine has termed this "Varmitphobia."
He has never run off like this with anyone on him, but it makes my mom (who is almost 50) nervous to be on him outside the round pen. This is not a slow horse when he gets going, and with how powerful he is the best thing you can hope to do is hold on. I totally don't understand where this comes from, because he is 99.9% bombproof except for this. I've seen him run away really fast in the pasture, and I know he's seen another axe murderer.
Does anyone have any advice as to how I can get him over this? Or at least stop him once he goes? He's 16 years old, I've seen him stare down a Brangus bull and not flinch, but this is crippling to our trail riding habit.