Fillies

When Genuine Risk ran in the 1980 Preakness she became the 50th filly to start in the race since 1873 but the first of her sex to compete in all three of the Triple Crown races. Genuine Risk compiled the best record for a filly in the Triple Crown, winning the Derby and running second in the Preakness and Belmont Stakes. Eight years later in 1988, Winning Colors, the Derby winner and 51st filly in the Preakness also ran in all Triple Crown events. In 1999 Excellent Meeting became the 52nd filly to start in the Preakness after finishing 5th in the Kentucky Derby.   Besides Genuine Risk and Winning Colors, only two other fillies have competed in more than one of the Classics, Careful in 1921 and Nellie Flag in 1935.

A filly is a female horse, specifically one which has not reached sexual maturity. This usually means it is under 24 months of age. After it reaches maturity it is
known as a mare. Most horse owners, however, agree that "filly" means a female horse under the age of 4 years old, while "mare" is for a physically mature female
horse, which is age 4 or above.

In thoroughbred horse racing, the definitions of "filly" and "mare" are slightly different. In this context, female horses are not classified as "mares" until they are
five-year-olds-more precisely, on the fifth January 1 (for horses born in the Northern Hemisphere) or August 1 (for horses born in the Southern Hemisphere) after their actual date of birth. Harness racing sets the cut off age for "mares" as four years.

The equivalent male is a colt. When horses of either sex are very young, they can also be referred to as foals.