My horse was handled pretty rough in his formerlife and was extremely spooky at everything. The the point of riolling his eyes and rearing up when he saw a saddle or western hat, basic exposure training and being nice to him help, but you can't think of everything, I have done some trining drills that don't keep him from getting scared, but have changed his reaction, In the round pen on a 12 foot lead line, I intentionally scare him, wave hats, rocks in a beer can, plastic bags, I change all the time. As soon as his feet stop moving I stop waving item. Then do it again, each time as soon as his feet stop I stop. Basically I have taught him to freeze when he sees something spooky instead of rearing and spinning 180 unexpectadly, On the trail he will now freeze, I can collect him encourage him a bit and usually get him to go on by without incident.