Season 2, like Season 1, has the kids making sense of the monsters through the Monster Manual book of their favorite game Dungeons & Dragons.
Scarily enough, I still have this book. Got this in, oooohhhh, ummm, 1979 or so. My friends and I played this game constantly for years. It's amazing this book still have a cover and the pages inside.
Here is the inner cover page. Whew-scanning is fun.
Okay, in the first season, they were calling the monster the Demogorgon. Here is its entry.
And in Season 2 they call another monster the Mind Flayer. Here is its entry. I think we briefly are shown this page during one of the episodes.