Cthulhu

Cthulhu is a fictional cosmic entity created by H.P. Lovecraft. In Poptropica, he appears in both a Pop quiz entry and is written about in Poptropica: The Text Adventure!, the April Fools joke posted on the Creators' Blog. There, he is a described as an elder demon who exists in the "Chamber of Elemental Evil".

Poptropica: The Text Adventure!
In the Chamber of Elemental Evil, Cthulhu arises in front of you. One of the tentacles snaps toward you. You decide to jump over it, but while you spend time deciding, it whacks you. You fall but regain your strength. You open your inventory and equip the Demon Blaster. Blasting the demon, it emits an "elemental cry of fear, pain, and rage". You blast it two more times, destroying it. Small fissures appear throughout Cthulhu's body, and white light emanates from within.

Pop quiz
Cthulhu appears in the Pop quiz "What is the scariest monster?" as the last option. In the picture shown, he is destroying and terrorizing a city.