The sea-bound god monster who rules over the Deep Ones. Ancient Philistine legends speak of Dagon, The Fish-God. His Pacific abode was discovered by the supercargo from a freighter sunk in World War I who escaped his captors only to find himself on the suddenly risen land once ruled by Dagon. Mentioned also as "Father Dagon" to "Mother Hydra." It is possible that this is the same land which the submarine U-29 found in "The Temple." Dagon, the chief god of the ancient Philistines and later the Phoenicians, represented as half-man and half-fish. One of the stories of Dagon from the mediaeval grimoires was that he placed the Ark of Jehovah in Dagon's temple when they had captured it from the Israelites. The force of the Ark destroyed Dagon and hew his statue to pieces.
Lovecraft may have come upon Dagon through Herbert S. Gorman's novel. In "Supernatural Horror in Literature" Lovecraft writes: "A less subtle and well-balanced but nevertheless highly effective creation is Herbert S. Gorman's novel, 'The Place Called Dagon,' which relates the dark history of a western Massachusetts back-water where the descendents of refugees from the Salem witchcraft still keep alive the morbid and degenrate horros of the Black Sabbat." He may also have come across it in Masonic literature since Dagon is highly mentioned in the hierarchy of demons there.
Theologically, Dagon was a vegetation god in ancient Mespotamia. His name meant "corn." Dagon was portrayed half man and half fish. He is said to have sired Baal while in the guise of El. Dagon is one of the oldest of man's gods. The Ras Shamra texts describe Dagon as coeval ("of the same era") with El, who is the most ancient and senior of all the Semitic Gods. Dagon's temples were in Philistine for about 2000 years, although Baal took over in most parts of the Middle East. Dagon's temple at Ashdod existed right up until the time of the Hasmoneans [who ruled parts of Palestine in Jesus' days]. He was consequently accepted and modified by Christian theology as a demon and embraced by the Masonics as a high angel. [Middle English, from Latin, from Greek Dagon, from Hebrew Dagon, "small fish," diminutive of dag, fish.]
("Dagon," "The Shadow Over Innsmouth")