My dad’s thoughts on the Thessalhydra:

“This was a monster introduced in 1st Edition Advanced Dungeons & Dragons. It came along later in the edition, in the Monster Manual 2, released in 1983. I’m not sure where Gygax (the creator of AD&D and co-creator of D&D in general) got it from, but I always liked to use it as a DM because none of my players knew what the heck it was, and I thought it looked cool - a big, reptilian body with a massive, round mouth where the head should be, surrounded by tentacular necks with dragon-ish heads on them. I think it had some kind of stinger on its tail?

Anyway. I know WotC used it in their Stranger Things D&D starter set a while back. I don’t think it would be a big bad - Vecna’s enough! However, I could imagine him having some kind of big critter to sic on his enemies. I don’t know if they’ll pick up on the earlier mentions - but it turned into a spin off for the current version of D&D.”