The Filly (horseracing) reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)
In thoroughbredhorse-racing, a filly is defined as a female horse less than five years old; in harness racing a filly is a female horse less than four years old. The male equivalent to a filly is a colt. An older female horse is called a mare.