A filly is a young female horse. The male equivalent of a filly is a "colt."
In most places, a filly is not yet four years old, although sometimes a female horse younger than five is considered a filly. Before she is a year old, a filly is also called a "foal," or a baby horse — and between the ages of one and two, she's also a "yearling." Around 1400, the word was spelled fyly, possibly from the Old Norse fylja, the feminine of foli, or "foal."