I've seen a lot of Cthulhu questions on the Sci Fi stack exchange, would he be considered on topic here?
He would be considered a mythological creature wouldn't he, the same as The Minotaur or Medusa?
I have to disagree here. All mythologies are fictional. What makes Lovecraft distinct among modern mythologies is the degree to which his ideas have been consistently adopted by modern artists. I see references ranging from horror classics like the Evil Dead, to children's films like Pacific Rim.
But this is just my humble opinion.