Thessalhydra - Huge Monstrosity (CR 20)

"It's the Thessalhydra, I'm telling you."
"It's not the Thessalhydra!"
"I'm telling you, it's the Thessalhydra."

- Stranger Things; Series 1, Episode 8

Ah, the Thessalhydra - one of my all time favorite beasts from the AD&D 1st Edition Monster Manual II. A towering nightmare, that comes somewhere between T-Rex, a nest of snakes and a bath of acid, it used to terrorize my players back in the day almost constantly. 

Thessalhydrae would be expanded in 2nd Edition to be just one of a whole crew of "Thessal-" monsters, including such crazy hybrids as the Thessalgorgon and Thessaltrice. However, the best is the original, and here, for 5th Edition, are my stats for that original badass.