Furious

The Beast is the scariest, ugliest, and ferocious of all dragons. Its species are unknown. It is believed to be a bigger relative of the Seadragonus Giganticus Maximus. It only appears in How to Break a Dragon's Heart.

In How to Break a Dragon's Heart
The Beast is the main antagonist of the book. He appears when Fishlegs is kidnapped by the berserk tribe and they attempt to use him as a food for the beast. However, Hiccup, Toothless and his friends come to save Fishlegs. After a long battle, they finally manage to kill the Beast and defeat the Berserks.