Some minor details to add on, I've also encountered archydra pretty late in the game, which are pretty much a reskinned hydra, just tougher and spits blinding poison. Luckily, that one is pretty easy to reach, once you get sufficiently far in the story.
Of particular note, though, one situation I have yet to replicate effectively; it seems there are times when the hydra use their grabbing bite, that you get caught by your legs, and don't even get to struggle, just get tossed right into the air and gulped without a fight. This has happened both to my character and my main pawn. I'm guessing it occurs if they grab you midair from below, but I can't quite get it to happen reliably just yet.