He was the king of the Lapiths and son of Ares in Greek mythology. I don't know if he became a God or something.
Dante somehow made him a guardian of the fifth circle of hell.
It's important to understand that Demon doesn't mean an "evil" or "scary" being in Satanism, if that's what you have in mind. Demons are beautiful beings, and they love humanity. They are kind and generous with those Satanists they guide. It's the enemy beings, angels and greys that hate humanity and which are behind Christianity and other anti-life religions.