Regarding Cthulhu:
http://en.wikipedia.org/wiki/Cthulhu
Wikipedia article

http://en.wikisource.org/wiki/The_Call_of_Cthulhu
HP Lovecraft's original short story.

You probably only want it if you're going to playing the Call of Cthulhu RPG, however.

As for me, I have various monsters (all gif, sized for board avatar):

Illithid (rubbish compared to Nemglam's)

Doppelganger wizard