What is a stallion?

A stallion is a fully grown male horse (over 4 years old, before that age he is called a colt) who has not been castrated. A male horse that has been castrated is called a gelding, and most male horses are gelded because stallions are often unpredictable and moody. Most stallions are only left intact (not gelded) because they are wanted for breeding. There is no other reason to leave a horse entire.