Leviathorgan

The Leviathorgan is a huge, ancient sea-dragon that lives in the ocean. It has large, glistening black wings that it uses to glide with. It first appears in How to Ride a Dragon's Storm. Apparently, it does not ever stop growing, and is said to grow a centimeter every day. Its greatest weapon is the enormous barb located at the back of the roof of its mouth that it can use to petrify its victims.

Hiccup killed one by using Norbert the Nutjob's axe as a lightning conductor, and the lightning then went through the ship to the Leviathorgan.