My story is set in pre-modern Japan, circa 1785. I have researched old Japanese horse breeds and riding tack. I have also googled "horse behavior", "horse bolting", "horse spooking", and a few other related terms.
What I have in mind for the scene: It's night, on a narrow path in the woods. There are three men, one girl and two horses involved. At first, two of the men are mounted and another man and the girl are on foot. Everyone present is emotionally agitated and tired. The horses are ordinary riding horses, not trained for warfare. They were ridden fairly fast for about ten miles earlier in the day, but have had a good rest since then. They were probably fed and watered within the last few hours. They have also witnessed some fighting and bloodshed.
One rider dismounts from the horse in question and offers it to the man on foot, who accepts. He mounts awkwardly, since he has the use of only one hand and hasn't ridden for some time. He was trained (he's a samurai) but doesn't like riding or horses in general. He then attempts to give the girl a hand to get up behind him.
(Or would it make more sense to want to put her in front of him? This is an antique Japanese saddle, which has moderately high wooden arches fore and aft, and is secured by a belly band, a breastband and another set of straps that cross on the hindquarters and go under the tail. The girl is an adolescent rather than a young child, so she's not so small that she could be held in the lap like an infant. They need to go about six or eight miles.)
The girl is reluctant to mount, being a little afraid of the animal, and stands back. The newly mounted man attempts to start off without her. The dismounted man seizes the bridle or reins to stop him, which starts an argument. The other mounted man, who hasn't stirred from his horse, is particularly angry, though he doesn't join in the shouting. The dismounted man insists that the girl get up on the horse. So he picks her up and plops her behind the saddle. She has to grab hold of the rider and thrash around to get one leg to the other side of the horse so she won't slide off. She's now panicked, though not screaming. The dismounted man smacks the horse on the fanny, and it takes off down the trail at full bolt. The man in the saddle is unable to do much more than hold on, since he's encumbered with the girl and has only one hand to control the reins.
Make any sense? I assume that the horse would already be on edge because of the emotional atmosphere around it, and that being badly mounted and having its bridle seized would upset it. Then the shouting starts and the girl is suddenly thrown onto its back. Would it be most likely to just bolt at that point, without a smack? Would it already have tried to dump its rider or bite someone? Basically, would most horses be able to hold on that long, or would they have cracked much sooner if they were going to crack at all? It's not actually essential that the horse be in full panic mode, but it does need to run a reasonable distance before the rider can slow it down.
From what I've learned about Japanese breeds, they are small (10-12 hands high in this period) and have a lot of other pony-like characteristics, like broad barrels and big heads. I don't know if that extends to a calmer temperament, or if it was common to geld horses for riding.
Also, since it's night (only starlight and a crescent moon besides lanterns) would a horse outrun its own eyesight? I am told that horses see somewhat better in the dark than humans do, so how likely is it that the horse could avoid any tripping obstacles in the way, like tree roots? Would it even care at this point?
Any information, informed speculation or links are welcome, especially if you can point out factors I've overlooked! Thanks for reading this very long post, in any case. :)