A female horse, of course
[DOC] [TLE] Best Race Horses of All Time - Top Ten List - TheTopTens [PAR] Secretariat was the fastest horse ever, & since the topic is "The best race horses 
of .... Perhaps he lacks the stamina for the Derby over a slightly longer course, .... 
BEST racehorse in America (not counting Zenyatta, she is so the best female).