He can be any age or gender, although many hot horses are mares, because hormones seem to create a certain amount of unpredictability or volatility. He's usually a breed with more "blood"--a Thoroughbred, Thoroughbred-cross or one of the lighter warmbloods like a Trakehner or Selle Français. He's fairly thin-skinned, with a beautiful, shiny coat that doesn't seem to grow much, probably because his metabolism is high. He's often lighter-bodied with quick, agile movements--the epitome of what we think of as a "flight animal."
He's not particularly scopey and powerful, but he is extremely athletic, and with his lively movement, he has a very agile way of getting his body out of the way of jumps. He also has great foot speed--a real asset in a jump-off. He doesn't like to be touched or handled much. He prefers a quiet rider. Any kind of crashing around on his back, in his mouth or on his side will set him off. He was born this way and not made, although bad experiences and poor handling can make him hotter and "crazier" in short order.
He tends to be more physical than cerebral, so he overreacts to sensory stimuli--which can result in oddball accidents, but will also make him very careful. If he were human, he'd be creative, artsy and highly talented, but half-crazy. He'd probably have attention-deficit disorder and not be all that good at doing two things at once. But the one thing he can do, he does well. He rarely suffers fools gladly, but he's still good-natured. You just have to be a little more sophisticated and thoughtful about finding ways to get along with him.
To read more about Callan's system for normalizing your hot horse's life--and yours--see the article "Chill Out Your 'Hot' Horse" in the July 2008 issue of Practical Horseman magazine. Watch a video of Callan's exercise for increasing his carefulness.