Chiron

Of course, Archer had a human form and lacked the lower body of a horse that should be the proof of being a centaur. However, that wasn’t anything to be surprised by. After all, Chiron was born from a god and a goddess, and was a great sage who was extremely close to being a Divine Spirit.

He could probably freely take the form of a half-horse and the form of a human at will.

…Yeah, if Archer was Chiron, then it also made sense how he had wounded Rider of Red who possessed divinity. Also, Archer’s great depth of wisdom that seemed to apply to everything was more characteristic of Chiron than Paris

- Vol 4 Chapter 2