Respectfully dissenting from the other replies here. Seeing as the demogorgon was written about 1600 years ago, it’s safe to say the creature is in the public domain. Not the Stranger Things version of the creature design specifically, but the name and any attributes derived from the older mythology are ok in terms of licensing.
https://en.m.wikipedia.org/wiki/Demogorgon
You could, but that’s not to say you should. As shown by the other replies here, people do strongly associate it with Stranger Things as that popularised the creature in recent times, although it’s mentioned in a lot of older literature (ie. Paradise Lost, Moby Dick, Dungeons & Dragons resource books). Regardless, straight up SCPifications of creatures from popular media or mythology tend not to do well here, as evidenced by deletions logs. Unless you had a particular idea to subvert or expand on aspects specific to the demogorgon’s mythology, probably better to avoid using it.