What Makes a Horse a Filly

What is a filly? This is just one of several words used to describe a horses age and gender. The word is used to differentiate between a young male and female horse. All baby horses are called foals but a filly or filly foal means that the foal is female. 
Definition of Filly 
A female horse under four years of age is called a filly. A female horse over the age of four years is called a mare. The plural of filly is fillies.
Characteristics of Fillies
Many people feel that fillies are born with a bit more intelligence than colts. Some believe theyre quicker to gain their feet and start to nurse. Whether this is true or not is debatable.

Colts however may mature faster than fillies physically especially if gelded at a young age. Some also believe that fillies are easier to sell than colts largely because when buying a young horse that is ungelded the new owner must commit to the extra expense of castration. Fillies and mares can be spayed but it is very unusual and a far more complicated procedure than gelding
Incorrect Usages of Foal Colt and Filly
It is common for nonhorse 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 threeyearold filly but it wouldnt be uncommon to hear a young female horse referred to as a young mare even though this isnt 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 arent 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 MerriamWebster 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.