There's one monster that is both iconic & served as my introduction into Advanced Dungeons & Dragons and no its not a dragon. Instead it was the Hydra one of the most underrated AD&D monsters ever. Hydras are not only dangerous but down right deadly.
One of the first AD&D games that I got involved with had to do with a particularly nasty dungeon in a stinking fen. The dungeon was partially submerged and we came up against a Hydra that was using the ruin as a base of operations to terrify the locals.
It wasn't until almost two months later that we found out that the hydra was the pet of a local necromancer and used to populate his army of undead by raiding the local swamp farmers.
One of the things that I figured out really quick was that there was definitely a respect that we developed for the hydra as a monster. The creatures are incredibly deadly and that's before the regeneration of the heads.