What creature is the main monster in Stranger Things season 1?

The Demogorgon is the main monster in Stranger Things season 1.

The characters borrow the name from Dungeons & Dragons to describe the humanoid creature emerging from the Upside Down. It has a flower-like head that opens to reveal rows of teeth, making its appearance one of the show’s defining horror images.

The season’s Demogorgon is connected to the breach at Hawkins National Laboratory and to Will Byers’s disappearance. It hunts across Hawkins and the Upside Down, while Eleven eventually confronts it at Hawkins Middle School.

The name is not a precise scientific identification made by the laboratory. It is a nickname chosen by the children because the monster resembles a dangerous creature from their role-playing game. Later seasons introduce other threats, including the Mind Flayer, Vecna, and Demobats, so those names should not be used for the season 1 monster.