In Season 5, the gang will fight Vecna, Demogorgons/dogs, bats, and the Mind Flayer. As well as a new monster, the Thesselhydra. They've foreshadowed it since the first dungeons and dragons scene, and in S4 when Nancy is describing the vision from Vecna, she describes a new monster with a gaping mouth. If you look up images of a Thesselhydra, its exactly that.