The Big HorseThe big horse, in racing vernacular, is the animal that brings fame and fortune to a stable. He's the heavyweight champion, the All American quarterback, the four legged Michael Jordan of the barn. Seabiscuit was once Tom Smith's big horse. A generation ago, Secretariat was Lucien Lauren's. In 2003, Funny Cide was Barclay Tagg's. In sixty years as a trainer, P. G. Johnson had never had one until Volponi. P. G. Johnson was a blue collar wizard, a