Chiron, in the original Greek myths, was the son of Kronos, but not a god. He was made immortal by the Gods, so he could teach heroes and train them until the world did not require his services anymore. Only then would he die.
In Percy Jackson and The Olympians, Chiron was first disguised as Percy's Latin teacher, Mr. Brunner, at Percy's school. He pretended to be handicapped, but the wheelchair he used was magically designed to hide his equine half. Later he was revealed to be a counselor at Camp Half-Blood.