What Makes a Horse a Filly

It is common for non-horse people to call all baby horses colts, but this is incorrect. A female baby is a filly and a male is a colt. Using the word "foal" is the correct way to refer to a young horse of unknown gender still with its mother. A young horse can be referred to as a filly foal or colt foal.

Once a filly is weaned, it may be called a weanling filly, and when it reaches the age of one, it may be called a yearling filly. After that age of 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.

In English, a slang way of saying girl may be to refer to her as a filly. In Australia, Champion Fillies day is celebrated with entertainment, food, fashion, and racing.