The Demogorgon is a grotesque creature from a parallel dimension known as the Upside Down.
The Demogorgon is mostly humanoid in stature and body shape but with thin, fleshy skin with bones visible. Some body parts, such as the arms, legs, fingers, etc. is more elongated, making it appear taller than a normal human. One of the more noticeable aspects of the Demogorgon is its massive gaping mouth with circular rows of teeth and several jaws that, when together, resemble the shape of a butterfly or flower.
The Demogorgon has the ability to travel to and from Earth and the Upside Down by using rifts that it is able to create. Much like the other creatures of the Upside Down, the Demogorgon has a sort of hive mind connection with the denizens of the Upside Down, such as the Mind Flayer, demodogs, and the fungal and flesh-like environment that it inhabits.
Even though the Demogorgon was a major antagonist in season one of Stranger Things, it's origin remains mostly unknown because of the single Demogorgon being the only one present. Despite its method of reproduction being impregnating human hosts with slug-like larvae, the larvae turn into demodogs than demogorgons. The demodogs do have a lot of similarities with the Demogorgon such as the mouth and skin, but are more quadrupedal than their biped counterpart. There is a possibility that the demodogs are an earlier stage in the life cycle of the Demogorgon, but there currently isn't enough canon information to dictate that being the case.
Since the Demogorgon is non-sapient, it mainly acts as more of an animal than a sentient creature. The way the Demogorgon hunts down its victims on Earth is reminiscent of how a shark hunts its prey. Like a shark, the Demogorgon can detect blood on Earth despite currently being in the Upside Down, once he detects the blood, it then travels to Earth and take its victim to the Upside Down where the Demogorgon will either eat its prey or impregnate it with it's larvae by keeping its victim in a sedated like state.
- The Demogorgon was named after the Dungeons & Dragons monster of the same name by the character Dustin in the show because of how another character named Will Byers was taken by the Demogorgon after finishing up a D&D campaign with his friends with the last monster they were fighting being the Demogorgon.
- The signature design of the Demogorgon was inspired by the monsters created by John Carpenter, such as The Thing.
- The Demogorgon is very different from the D&D creature of the same name, sharing almost nothing in common with it. However, the kids use this name to identify the creature since they do not know what it is.