A filly is a young female horse that has not yet reached full adulthood, usually under about four years old. The male equivalent of a filly is called a colt.

Basic meaning

  • A filly is a female horse that is too young to be called a mare.
  • In most horse contexts, a filly is under four years old; in some racing rules, the upper limit can be four or five.

Age terms for young horses

  • From birth to under one year, horses of any sex are called foals.
  • After about one year, a young female is called a filly and a young male is called a colt.

When a filly becomes a mare

  • Around four years old, a filly is generally reclassified as a mare, meaning an adult female horse.
  • In some racing jurisdictions, a horse may still be officially classed as a filly in certain races up to age four or five.

Use in horse racing and breeding

  • Horse racing programs and results often group horses into races specifically for fillies because of age and sex differences in development.
  • Although many fillies are sexually mature by about two years, responsible breeding usually waits until the horse has mostly finished growing, around four or five years.

TL;DR: A filly is simply a young female horse, typically younger than four years old, before she is old enough to be called a mare.