I found it by exploring after killing the Bell Tower Gargoyles. It's in like a magical forest or something. If you're looking at the front of the Bell Tower building, you go to the right of it. Then you go down a tower where there's a blacksmith, and you run past this creepy giant crawling thing, and then just keep going in the forest-y area. Sorry I can't really be more descriptive, I'm not very knowledgeable of the game

So you're talking about the first hydra in Darkroot Basin?

Uh probably I guess