A baby horse is called a foal, with gender-specific names being colt for a male and filly for a female. Foals that are still nursing are sometimes called sucklings, and a weanlings after the nursing period ends.
When a foal reaches 1 year old it is called a yearling. When a horse reaches its second birthday, it is typically referred to by its adult name: mare for females and stallion for males.
Baby horses are often mistakenly called ponies, but this is incorrect. Pony is the name given to horses that are small in size but are still full-grown. Ponies can be ridden, while foals cannot.