A filly is a female horse that is too young to be called a mare. Here are some key points about fillies:

• A filly is generally a female horse under the age of four.
• In some nations, such as the United Kingdom and the United States, the term "filly" may be used for female horses under the age of five.
• The equivalent term for a male horse is a colt.
• When horses of either sex are less than one year old, they are referred to as foals.
• Horses of either sex between one and two years old may be called yearlings.
• A female horse over the age of four years is called a mare.
• The plural of filly is fillies.
• A female foal can be referred to as a filly foal when it is first born.
• Young donkeys, ponies, mules, zebras, onagers, and other equine relatives may also be referred to as fillies.
• The term "filly" describes the horses gender and age.

In summary, a filly is a young female horse that is not yet a mare.