Yeah. I've encountered this before. Zombie Hydra. And I didn't even look in legends....
This was one of my more memorable worlds. I forget what it was called, but there was this place called the Blizzards of Ooze. And it was baaaaaaad.
Seriously. I tried six separate times and failed each time to establish a fort there. It wasn't just evil, I swear the region itself was cursed. I think the world is still up on DFFD actually.
Anyway, I was dicking around in adventure mode, and I eventually ended up down there. Found a lair, went in. Nobody was there. So I shrugged and turned to leave... and ran into the zombie hydra on my way out. And pretty much crapped my pants.
I managed to kill it, and I butchered it to make sure it stayed down. Went to sleep to recover, and I woke up to see that thing staring me in the face. Since you can't tan in vanilla adventure mode, the skin had reanimated while I was sleeping.
I proceeded to run from it, all the way across the Blizzards of Ooze, with a wounded leg, dodging at least one cloud of nasty weather. (From observations on a nearby pack of giant birds, it wasn't a thralling cloud, but inflicts some kind of sickness. Wasn't sure what, I managed to avoid. It certainly wasn't insta-death, though.) I eventually got all the way outside the evil area, then chopped off each of its heads before it finally went down. I then put each piece into my backpack, slowly dragged my way to a shop, and dumped the pieces of scale on the shopkeeper by including it in part of a trade.
Single-handedly my greatest adventure mode escapade. (Mostly because my others involve just getting ganked by goblins or something stupid. But still.)