Most horses that run (and win) in the triple-crown races – Kentucky Derby, Preakness Stakes, Belmont Stakes – are colts. Colts are male horses under the age of four. Past four, male (adult) horses are called “stallions" or just plain old “horses." All 11 triple-crown winners have been colts.

Fillies are female horses usually under the age of four, but for the sake of horse racing, that limit is pushed up a year to five. Adult female horses are called “mares."

Since the Kentucky Derby is a race for 3-year-old horses only, no mare has ever won. However, three fillies have won the Kentucky Derby: Regret (1915); Genuine Risk (1980); and Winning Colors (1988).