Male horses are born colts. Upon castration, they are known as geldings. As the colt ages and reaches age five, he'll be known as a stallion. Geldings are geldings as soon as the procedure is complete, no matter the age.
When a male horse is a baby it's called a Colt. When the horse reaches a year it's called a Yearling. Around the age of two it's either gelded or it remains untouched. A gelded horse is called a Gelding while an in tact horse is called a Stallion.
