A female horse that is younger than 4 years of age is called?

A female horse that is younger than 4 years of age is referred to as a filly. This term is specifically used to describe young female horses before they reach maturity. The classification between a filly and a mare is primarily based on age, with "mare" denoting a female horse over the age of 4. Understanding these terms is important for those involved in equine care, breeding, and management, as it helps in identifying and categorizing horses appropriately based on their developmental stages.

Other terms like stallion and gelding pertain to male horses; a stallion is an uncastrated male while a gelding is a castrated male. These terms reflect the sex and reproductive status of the animals, further clarifying why they do not apply to young female horses.