Cthulhu first appeared in the "Treehouse of Horror XXIV" intro. He was seen while Bart was skateboarding home. His tentacles created a pathway for Bart's journey home. Cthulhu also shared cups of tea with H. P. Lovecraft himself.
Cthulhu also played a major role in The Simpsons' Treehouse of Horror #19. In the story, Skinner has had enough of Bart, and tells him and Milhouse to clean up the school's old library. While Milhouse gets to work, Bart looks for something interesting. He soon finds the Necronomicon. They then use the old books to build a staircase to escape through a school window. As they go home, Bart realizes that the Necronomicon is the book of the dead, and tells the history of the monster gods called the Great Old Ones. The book also shows them how to summon them and make them do their bidding. Bart decides to summon Cthulhu to get revenge on Skinner. At first nothing happens, but slowly as they walk around town, tentacles appear to kill and mutilate people. These included Jimbo, Kearney and Dolph. They discover that they really did summon Cthulhu, and decide to make Cthulhu attack Skinner. Cthulhu attacks Skinner at Bart's will. But soon Cthulhu was out of control, and was attacking everyone on sight! Bart and Milhouse go home to try and figure out how to get rid of Cthulhu using the Necronomicon. Santa's Little Helper got the book first, and transforms into a hellhound and then a squid-like monster. Bart can't find the answers in the book, and suddenly a tentacle knocks him out. He wakes up and goes down for breakfast, and finds that everyone has been turned into a minion of Cthulhu and possess many squid-like traits. They are also eating Milhouse for breakfast.
Cthulhu made his second TV appearance in the "Treehouse of Horror XXIX" intro. When the Simpson family travel to the strange town of Fogburyport, they discover that everyone living there are actually fish-like humanoid creatures who worship the evil sea god Cthulhu. They plan to sacrifice the Simpsons to Cthulhu, however Homer instead challenges him to an eating contest. Homer manages to win the contest, so Cthulhu concedes his defeat and offers to grant a wish for Homer. But Homer simply asks Cthulhu to allow his family to eat some of Cthulhu's body parts.
- THOH – "Treehouse of Horror XXIV"
- Episode – "Diggs" (name seen)
- Comic book – The Simpsons' Treehouse of Horror 19
- Video game – Tapped Out (THOH XXIX design used for Cthulhu, THOH XXIV design used for his son M'nthster)
- Episode – "Halloween of Horror"
- THOH – "Treehouse of Horror XXIX"