If you are relatively new to the horse world, you may be a little unfamiliar with some of the terminology, especially terms that apply to horses of specific ages and gender.
Many equestrians will call any young female horse a Filly, regardless of its exact age, and they are not really wrong. Once a female horse reaches age 4, they are should be referred to as a mare instead of a filly.
A Filly is a Female Horse Younger Than 4 Years Old
No, a Filly cannot be a Colt. The term Colt is the male equivalent of the word filly; therefore, Colt can only be used to describe a young male horse under the age of 4.