Dagon, also known as Father Dagon is a Great Old One and the consort of Mother Hydra, together they rule over the Lovecraftian race of the Deep Ones. He is worshiped by the Esoteric Order of Dagon, a secret cult that is located in Innsmouth.
Dagon is depicted as a gigantic Deep One (around 30 stories tall) and first appeared in the short story "Dagon". Since then he and his consort have more or less been established as the official rulers of the Deep Ones, though Hydra seems to gain a slightly larger following.
Lovecraft most likely based Dagon on the ancient Mesopotamian deity of the same name, who was a god of fish, fishing and fertility.