An adult male horse, if left intact, is called either a "stallion" if used for breeding, or a horse (sometimes full horse); if castrated, it is called a gelding. In some cases, particularly informal nomenclature, a gelding under four years is still called a colt.

At what age does colt become stallion?

Even though spermatozoa production begins as early as 12 to 14 months of age in most colts, results of numerous research trials recommend waiting until the stallion is 3 years old before using him as a breeding stallion. Daily spermatozoa production is also influenced by season.

What does a colt grow into?

When young horses reach breeding maturity, the terms change: a filly over three (four in horse racing) is called a mare, and a colt over three is called a stallion.