QUOTE(Jonathan Chin @ Wed 17 Sep 2008 0302)

Any equestrians here? How hard is it to train a horse to be aggressive? I remember from an essay by Montaign in the 16th century that some knights trained their horses to bite and kick. I presume having a horse prone to go berserk has its liabilities but better than one that gets scared easy.
I would not say I qualify as a genuine equestrian, as I do not own horses, have never owned horses, and don't work with horses on a daily basis. But I've been riding horses on-and-off for some 40+ years, and have been trained to ride and care for them to at least a minimal extent.
It is not very hard at all to train a horse to be aggressive. In fact it is often far more difficult to train them to NOT be aggressive. But it depends a great deal on the horse.
Very seldom does one see stallions (adult males) ridden by anyone of less than advanced riding skills. Most "riding" horses are either mares (females) or geldings (males castrated before adulthood). As with most herding ruminants, the adult males can be very aggressive. They are quite naturally a handful. Not quite as bad as a bull or buffalo, but still aggressive.
Much of the natural aggression is bred down, and the remaining aggression is usually trained down. But it still lies close below the surface. Usually the horse will learn through its training what is, and what is not, acceptable behavior. They are reasonably intelligent creatures (less so than a dog, but well above a cow or deer). Many are clever enough to figure out how to skirt around the rules just a bit, and I have experienced some moves by horses that were surprisingly aggressive, while not being overtly hostile. The males often try games to assert their dominance/control -- like trying to walk a rider off their backs by passing under low branches or rubbing against fence posts.
Even if well trained and controlled, when you get the horse excited its natural aggressiveness may well rise up. And horses are excitable creatures. That is one reason I speak in terms of "spooking" a horse, rather than "scaring" a horse. Sometimes you go "boo" and the horse will spook, and run away. Sometimes you go "boo" and the horse will spook, and kick you into the next timezone, or trample you into mush.
They like to run, and the stallions like to compete and to win. This is one reason that a cavalry charge is/was hard to stop once it got started. You could keep the horses in line while you walked them, or while you trotted them, or even while you cantered them. But the moment you got them gallopping -- the moment you "gave them their lead", it was almost impossible to reign them in. From that moment on they wanted the charge as much as the riders. You might be able to direct them, but you could not run for a bit, then trot for a bit. Once running, they were all-in on the game.
The key to training the next level of aggression seems to have been experience. You will see in the writings of the age of horse soldiers the premium that was put on the "old warhorse". A horse that had been in battle was expected to know what it meant to charge, and to know what to expect once in the thick of things, and to participate in the attack as much as the rider did. As far as I understand it, the process of training a warhorse was really a matter of selecting an aggressive stallion to start with, working to teach when he was under control vs. when he had his lead, and then giving him some experience in combat.
-Mark 1