Doomfang

The Doomfang is a very large sea serpent said to be as black as one's nightmares with very large teeth and claws. It also breathes a strange blue fire that freezes the enemy. The only known Doomfang lived in the Wrath of Thor (a straight between Villainy and Hysteria) for near fifteen years before moving on into the open ocean.

In The Books
Before the events of the book series, the Doomfang lived in the Wrath of Thor. When Norbert the Nutjob's father, the chief of the Hysterics, returned from a trip to America, he was killed by the dragon. Norbert the Nutjob swore to take revenge on it using the arrow found sticking out of the potato his father had found.

In the How to Cheat a Dragon's Curse, Hiccup and Camicazi steal the same potato from Norbert the Nutjob (the current Hysteric Chief) to cure Fishlegs of Vorpentitis, a sickness caused by the Venomous Vorpent sting that is otherwise incureable. While making their way back to Berk after the ice had thawed, the Doomfang breaks through the ice and attacks the two for the potato because the Doomfang itself was suffering from Vorpentitis. It even leaves the Wrath of Thor while chasing after them and eventually does succeed in eating the potato, leaving Hiccup without a way to save his best friend's life. Despite swearing revenge on Hiccup for embarrassing him earlier in the book, Norbert the Nutjob allowed Hiccup and Camicazi to escape for having gotten rid of the Doomfang for the Hysterics.

In the epilogue of the book, a much older Hiccup says that the same Doomfang that stole his potato followed him around like a guardian angel every time he went sailing. All the other ships sunk while Hiccup's ship stayed afloat.

Facts

 * They have big, green, hypnotic eyes.
 * They normally avoid humans.
 * Their forked tongue can reasch up to 100 meters.
 * Their threat is high! Sail away!
 * Their sub-zero sea-blue "fire" freezes whatever it touches