What type of horse is specifically known as a female under four years old?

The term that specifically refers to a female horse under four years old is "filly." This designation is used to indicate the age and sex of the horse, distinguishing it from other classifications. A filly is typically considered to be a young female horse, and once she reaches four years of age, she is generally referred to as a mare.

In terms of terminology, "mare" refers to an adult female horse, and "colt" pertains to a young male horse, usually under four years old. The term "foal" is a more general classification that refers to a baby horse of either sex, typically under one year old, which makes it a broader category not limited to gender or age specificity like "filly" is. Understanding these terms helps in communicating accurately in equine contexts and assessing horses based on their age and sex.