A baby female horse is called a filly. Baby horses in general are called foals. A male baby horse is called a colt. Sometimes, the term “colt” is used incorrectly to refer to any baby horse.
A filly is defined as a female horse under four years old. Once it passes the age of four, a female horse is called a mare. The date that a mare is predicted to give birth is sometimes called a foaling date.
Babies of other species in the horse family are also sometimes referred to as foals. These include baby mules, donkeys, ponies and zebras.