I recommend this sticky thread (now at 139 likes): This is how we train a fearless trail horse!
"There's no time to react or one rein stop her
There generally is time, but not much. Sometimes there is a slow buildup of pressure, when you are going toward something the horse is aware of and not happy about. Sometimes it is pretty much instantaneous, like a garage door opening at a house, or an invisible pink rhino spotted by your horse. But even then, there is a brief moment where the horse gathers itself and either leaps forward or spins around. By some magical method I've never quite figured out, you need to be relaxed enough to give your horse confidence, but alert enough to catch the horse in that brief moment of time.
A hard 180 can help if done before the horse is fully moving forward. Lots of folks like a ORS, although I'm not a huge fan. I think switching to a curb bit helped me, but a lot of folks will tell you that means I'm a terrible rider/person. I think it also helps to work on your horse's stop while in a non-threatening environment. After all, if your horse's stop isn't immediate and correct when trotting in an arena, you can't expect him to stop well when scared almost out of his mind. Practicing stops by the hundreds and even thousands - and good stops, not sloppy ones - can eventually build up a habit of obedience that even fear won't overcome. If the horse has to think
'this means stop', then the response isn't automatic enough to help you with a spook. It needs to be such a strong habit that the horse will do it without any thought at all. In a true bolt, IMHO, horses don't think - but they will respond to very strong habit patterns.
At that point, when your horse stops in spite of its fear, it soon learns that the scary thing goes away when it stops...and thus it learns to stop instead of bolt. At least, that is my experience with ONE horse
who bolted a lot. Cherie's advice in the thread linked above is based on vastly greater experience. The first time I read it, I thought it was hooey...but that hooey keeps looking better and better with time...
...to me and 138 others!