Just like the 1 arrow Red Dragon kill (that happened to me too) the Black Hydra at the Ash Lake just decided to die by itself. After 2 or 3 unsuccessful attempts at killing it, I just ran past it and go to the dragon at the end of Ash Lake. I've cut the dragons tail and while returning the Hydra jumped across the sand and just disappeared, giving me 10k souls and 2 dragon scales. It was bored I guess..

lawl anticlimactic

Yeah when I joined the covanent I walked out of the tree and instead of a hydra got a bunch of souls and dragon scales. Maybe it's just there to guard the covanent.

Yup, the Hydra on mine killed itself as well. I too tried several times to kill it and being unsuccessful I just ran towards the other side. Once I made it to the other side I had hardly any health left so I healed up while it continued shooting at me. Waiting for it to pursue me, and not knowing what was inside, I just sat still while letting the tree block all of its shots. It eventually gave up, I guess, and jumped to it's death.

Happened to me too, except, I was just wandering around not even knowing there was a Hydra, then it spawned so i booked it as far away as I could and it died. Kinda made me mad.