This is not Cthulhu himself, but a representation of one of his kindred. Sometimes referred to as "Star Spawn," they are more or less described as an octopus-like race which descended from the stars.
"Another race - a land race of beings shaped like octopi and probably corresponding to fabulous prehuman spawn of Cthulhu - soon began filtering down from cosmic infinity and precipitated a -monstrous war which for a time drove the Old Ones wholly back to the sea..."
-H.P. Lovecraft, At the Mountains Of Madness
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