A colt is a young male horse, under the age of four. An older male horse is called either a "stallion", if left fertile, or a gelding, if neutered. A young male horse is considered a colt even if he is gelded. The verb 'to geld' refers to the process of neutering a stallion.
A young female horse is called a filly until age four, and a mare thereafter.
A group of colts is called a "rag".
In the wild, colts are kicked out of their herds when they become stallions by the stallion of the herd. When they are kicked out, they usually will be able to find a herd made up of horses just like him called yearlings. They stay with this band until they are mature enough to get their own herd and take care of it. Some stay in a "bachelor" herd and try to steal from other stallions.