Hmm, I had read that Dagon was based on an actual god, who I thought was sumerian, but it seems I was wrong.
Here's an explanation on the whole 'Cthulhu Mythos/Sumerian Gods' thing:
Is it true that the Cthulhu Mythos is derived from Sumerian mythology?
(tap)This particular information comes from the Simon Necronomicon, a book of ill-repute among most Mythos fans. Generally, when someone states that there is a connection, they have only read the Necronomicon and have little knowledge of either the Cthulhu Mythos or Sumerian mythology. The "Cthulhu Mythos" names in the Necronomicon (those on pp. xix-xx) do not turn up in any other known Sumerian texts, or anywhere else other than Cthulhu Mythos stories and articles.
Aparently you
can make correlations, but these are very supperficial.