What is "exceller"

Exceller (May 12, 1973 – April 7, 1997) is widely considered one of the best horses to race in the United States not to win a year-end championship. Despite his exemplary achievements as a racehorse, and his unique accomplishment in being the only horse to ever defeat two U.S. Triple Crown winners in the same race (and only the second ever to do so in his career), Exceller is now remembered more for the tragic manner of his death and the horse rescue movement it helped inspire.