What is a young male horse called?

A young male horse is referred to as a colt. This term specifically designates male horses that are typically younger than four years old, differentiating them from stallions, which are adult male horses that have reached maturity and may be actively breeding. The use of the term colt highlights this early stage of development, where these animals are still growing and have not yet reached full maturity.

While other terms in the choices refer to different stages or types of horses, such as a foal, which is a young horse of either sex up to one year old, or a yearling, designating a horse between one and two years old, they do not specifically identify a young male. A filly, on the other hand, pertains specifically to a young female horse. Therefore, the term colt accurately identifies the category of a young male horse, making it the correct answer.