Created by H.P. Lovecraft, Cthulhu is a fictional being within the the eponymous Mythos. The High Priest of the Great Old Ones, it sleeps in the sunken city of R'lyeh, waiting for the time to be right to awaken and ravage the world. Described as a gigantic humanoid form with a squid-like head and dragon wings, Cthulhu is perhaps the most iconic figure of Lovecraft's work.
Perhaps the most direct link to the Yellow Mythos is with Hastur itself. In some Mythos lore, Cthulhu and Hastur are said to be "half-brothers" (Hastur is sometimes portrayed as having a vaguely octopid form reminiscent of Cthulhu). In some stories it is indicated that the two Great Old Ones do not get along, resulting in a rivalry of sorts.