The Devil

Satan or The Devil is the (usually horned) personification of evil in Human mythology and religion.

Other, similar horned gods appear in the lore of other races such as the Draconians and the Kaleds, the latter having a horned god of war. (DW: The Satan Pit) Several beings seem likely candidates for the inspiration of this myth. The Fourth Doctor believed that Sutekh had called himself several names, including Satan (DW: Pyramids of Mars)

The Dæmons, a horned race from the planet Dæmos, also had similarities to the Human concept of Satan (DW: The Dæmons), as did the Greek Immortal Hades, a supposed lord of the underworld. (PDA: Deadly Reunion)

The Beast said that he was the origin of Satan, and all similar entities on all worlds. The Doctor referred to him as the truth behind the myth, although the Doctor himself found the Beast, in particular his claim to have existed before the universe, a challenge to his own belief system. (DW: The Impossible Planet / The Satan Pit) He did eventually refer to The Beast as "the devil" (DW : Planet of the Ood), although it is unclear whether he was referring to it being the inspiration for the idea of the devil or the actual devil himself, having possibly come to a conclusion of what The Beast really was.