Flashfangs are large black dragons. They are similar in description to the Night Fury, but they only appear in the book: How to Speak Dragonese.

In The Books

In How To Speak Dragonese, the Flashfangs were part of the Roman Colosseum show. After the Sidewinders had caught and killed all of the rainbow-colored doves, the Flashfangs entered the arena and quickly made short work of them. Shortly after, however, the arena was flooded for Hiccup's, Camicazi's, and Fishlegs's portion of the entertainment and the Flashfangs were killed because of the roman flooding the arena (Flashfangs can't swim.)