I find TBs spook for a variety of reasons but the most common one is because they are trying to evade the work you’re doing. Sometimes its behavioral – sometimes it’s saddle-fit – for the most part you have to set them up not to fail and the only way to do this is to thoroughly inspect if every piece of equipment (especially that western saddle) and every criteria you ask of him is fair.
My old OTTB used to spook when he was a baby and no amount of “circling hard” ever fixed it. Personally, I don’t believe in that method - it doesn’t really rectify the behavior and it just supplements their worry; now, on top of spooking, they have to worry about having half their face ripped off around a tight turn.
What I used to do when my horse spooked is ALWAYS use a verbal reprimand: “HEY!” works very well on several horses - they usually stop and theyre like wait, what? A firm “NO!” works too. Then, go back to what you think spooked them. Or go to the area they spooked, and thoroughly walk back and forth so many times that it bores them.
FWIW, you don’t want to supplicate their negative behavior with a negative reinforcement in this instance because for all you know they could be telling you something about them being uncomfortable: negatively reprimanding them will only make them worry more.
That being said, if you find there is no “good reason” for him to be spooking (i.e saddle fits perfectly, etc) then you need to keep him engaged. What I like to do on trail rides or outside of the ring when I’m hacking is do leg yields back and forth across the trail - keeps them busy, and it keeps them focused on me: you can also, if he’s “knowledgeable” enough, ask for contact - whatever you do, keep him engaged.
