A young male horse is called a colt, especially one which is less than 2 years old and has not been castrated. A young female horse is called a filly. Horses of either sex up to 1 year old are called foals.
A unisex term for horses between 1 and 2 years of age is yearling, with horses nearing 2 years of age called long yearlings. Castrated male horses of any age are called geldings, and non-castrated males are called stallions.
Age in horses is counted either from their actual birth date or from New Year's Day of the year the animal was born. This depends upon the breed and the organization the horse is registered under.