In season 1 of “Stranger Things,” the boys call the monster the Demogorgon. The name comes from the creature in their tabletop role-playing game, Dungeons & Dragons, which gives the children a familiar label for something they do not understand.
Mike, Dustin, Lucas, and Will use gaming language to interpret the supernatural events around Hawkins. Their real-world monster does not necessarily match every detail of the Dungeons & Dragons creature, but the game provides a useful framework for discussing its danger and behavior. The name also makes the threat feel like part of the boys’ shared imaginative world.
The Demogorgon is the predator associated with the Upside Down in the first season. It is commonly confused with later threats in the series: the Mind Flayer becomes central in later seasons, Vecna is revealed as another major antagonist, and Demodogs are juvenile creatures related to the Demogorgon. For the season 1 monster, however, the canonical answer is simply the Demogorgon.