There are several terms for male horses depending on their age and whether they have been gelded. The most common terms are colt, gelding and stallion.
A colt is a young male horse. The exact age the term stops being applied varies, but four years old is the latest a male horse is deemed a colt. After that, the horse is called a stallion when left intact, or a gelding when it is castrated. Stallions are sometimes called studs, especially if they are used for breeding. Although it is not a common term, some male horses are called ridglings. This means they have cryptorchidism.