Trading in horsepower is second nature to Nathan McCauley. For many years his father Ron owned a big Toyota franchise in Lexington, and McCauley showed a precocious flair for the same game when opening his own dealerships in Tennessee. And since quitting automobiles for Thoroughbreds, McCauley has discovered a couple of niches largely unexplored by those horsemen whose own transitions, from one generation to the next, tend merely to consolidate familiar commercial conventions. In fact, since opening River Oak Farm for business in 2019, he reckons to have bought and...