In theory, all horses are created equal. Then stuff happens. Between their genes, accidents of life, and the designs of those who raise and train them, they develop strengths and flaws. And we end up, like George Orwell's pigs, with "some animals more equal than others." After hundreds of years of observation, however, we have concluded that one half of the breed is born with an athletic superiority over the other. And we duly grant Swiss Skydiver (Daredevil) a five-pound concession for daring to square up to males in the...