Two years ago it was Barbaro after the Preakness. Saturday, it was Eight Belles, after a heroic second-place finish in the Kentucky Derby. Racehorses of extraordinary promise, which had to be put down just as their careers were getting underway. Twenty of Saturday's Kentucky-Derby entrants were descended from one horse, Native Dancer. Ankles are the tragic flaw of that extended family. Jon Weinbach is sports-business reporter for the Wall Street Journal.