Usually, a horse that bolts is either insecure, or afraid of pain. Unfortunately, those reactions are trained in right at the very beginning of starting the horse- incorrectly and badly. That’s why it’s such a hard behavior to correct.
Many big horses are unfairly ridden with strong aids, because riders are afraid of them, and they learn to bolt in anticipation of pain- usually in their mouths. Others are severely disciplined because they were unsure and stopped to have a look at something that frightened them. I’m sorry, but 99% of the people who “start” young horses have no business riding them, and ruin them in the first 30 days of training.
In my experience, ALL young horses take off at some point, maybe even a lot depending on their personality, and ALL young horse are afraid of something they see. This is to be expected, and the correct rider response is to let them look as long as they need to, even get off the horse, and reassure them, walk them towards the object, whatever you need to do to let the fear rush fade away before you go on. If they take off, so what? Just go with it, and laugh, and pat them, and even urge them on- guiding them in a big circle and letting them figure it out. Just a few rides like that and it’s all over, forever.
When you have a confirmed bolter, well, that’s now a big problem, and now you not only have to figure out why, you also have to retrain them.
Frankly, I don’t find anything odd or unusual about this horse’s reported behavior of taking off with a strange rider in a strange environment. If you knew nothing about his history, would there be all these histrionics? Because any horse in the world could react this way under the circumstances.
I’d do a couple of things-
if he has no brakes to begin with, I’d put a double on him, and with no nonsense at all, I’d put them on. That takes about 15 minutes, one time, and its done. I don’t care what kind of rider or what kind of hands, a horse needs to understand that whoa means whoa, absolutely.
If he has brakes, but he’s just scared, I’d put the mildest bit possible on him, and ride him with the lightest contact possible, but I’d RIDE him. I’d have a total game plan, keep him concentrating because I’d be concentrating, and vary it up constantly to keep his brain with me. Besides, its more fun! This is how professionals ride anyway, its the ammys that daydream and get into trouble. And yes, the walk is part of the work out, not a time to amble around. More riders get into trouble at the walk than any other time. I’ve been dumped at a walk and from a total standstill, and both time I’ve been hurt worse than at a gallop. Both times sitting without stirrups and gallywagging- learn from my mistakes!
I’d also take him out of the barn, and ride him in varied environments. In order to do this safely, you need an experienced horse and rider with you- the horse to give your horse confidence, and a rider who is experienced at starting trial or field hunters. I would also use a standing martingale.
At first I would pony him- with a long lead thru the bit and over his head to stop him absolutely without breaking the bridle. If this goes well for a few weeks, I’d free him, but always keep him in company. If possible, he should be with an experienced horse in front of him, AND behind him. Then I would start exercises to give him confidence that he can be alone and ok- by riding him a little in front, then behind, then to the left and the right, so that he learns he can leave and be ok. After a couple of months of this, along with his arena time, you should have made HUGE progress in overcoming his panic attacks, and forming a real bond with him.
NO horse will never bolt. My 20 year old FEI horse a couple of months ago was walking back to the barn after a lesson, and right next to the barn, took off without warning 20 feet from “home”. Who knows why? He’s a horse. But almost as soon as he started, he realized he was being a dork, and stopped, because he wasn’t afraid of me flipping out on him, so he didn’t need to now run away from ME.
Bolters almost always are running away from their riders because they are afraid of the consequences of their actions. Give them no reason to fear you, and they will keep their minds clear.
There is a 3rd kind of bolter- these are the horses who are not afraid at all- but have learned that they can intimidate their riders by running off. These horses are VERY dangerous, and frankly, need to be put down before they kill someone, or find a rider who can channel that aggression into performance, if they have talent. They are thankfully, pretty rare, but believe me, they exist. This horse, from your description, sounds anything but this.
Good luck and enjoy him. I think he will be totally fine with a rider who isn’t scared to death he might make a mistake.