I work at a renaissance faire, one of the largest in the country, one of the best. Every longstanding fair offers its guests the opportunity to watch a joust, a quartet of armored knights squaring off on horseback in dust and sunlight. The horses pound toward one another, breakaway lances shivering into splinters. A rider falls heavily, the instant calculated to thrill the crowd.
A joust horse is trained to a simple, one-dimensional skill: pivot and run. Armored riders sit deep, lean back, control the horse through neck reining and a bit of balance. There's no space between man and horse for subtleties like leg aids and seat aids. Leg armor makes that nearly impossible.
Joust horses must be trusting and indifferent at the same time. No two shows are exactly the same, but the routine never varies. Gallop. Pull up. Pivot. Gallop. Stand. Banners wave, crowds cheer, the sound system crackles. The horses wear caparisons, long, flowing trappings; some wear face armor. The challenges might undo an ordinary horse, but the joust horse accepts his work because that's what it is, his work. It has nothing to do with him.
To train a horse away from the joust, off of the joust, is to train him to be what he has never been: an ordinary horse. For my boy, only two months off the joust, "ordinary" doesn't yet exist.