What is a young female horse called?

A young female horse is called a filly. Fillies become mares at 3 years of age.

Unlike many other types of domesticated animals, such as dogs and cats, female horses do not typically get spayed. Instead, they reach sexual maturity and are kept away from stallions (unaltered male horses) unless the owner or handler wishes to breed them. A filly is typically smaller and weaker than a colt (a young male horse). The term "foal" refers to a colt or a filly before its first birthday. In horse racing, the animals are called colts or fillies because, even though they are sexually mature, they are still less than 3 years old when they begin to run.