What Makes a Horse a Filly

 two, it may be a two or three-year-old filly, but it wouldn’t be uncommon to hear a young female horse referred to as a young mare, even though this isn’t strictly correct.

Horse Racing and Riding Horses

In the horse racing world, the word filly has a slightly different meaning. In a race, a filly may be any female horse running under the age of five years old. The riding horse world rarely makes this distinction, partially because horses aren’t really ridden except in training before they are four years old. If a horse show class is specifically for fillies, the young horses will likely be shown "in hand" — that is, not being ridden.

Young donkey, pony, mule, zebra, onagers, and other equine relatives may be referred to as fillies as well.

Roots of the Word Filly

According to the Merriam-Webster dictionary, the word filly has been used in English since the 15th century. Its roots are in the Old Norse word "fylja" for female foal, that evolved to the Middle English word "filli" and is also similar to the Old English word for foal. The French word "fille" has similar roots.