A foal is a baby horse. Most horses give birth to only one foal at a time, though occasionally they have two.
You can use the word foal for a horse that's younger than one year old — after turning one, a foal becomes a yearling. Foals can be either male, also called a colt, or female, also called a filly. When a mare, or adult female horse, has a baby, you can say she foals. The Old English root word, fola, means "foal" or "colt."