Flashfangs are dragons, that only appear in the book: How to Speak Dragonese and the Complete Book of Dragons: Guide To The Dragon Species.
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).
Apperance
Flashfangs are thin, black dragons with long fangs, a hooked tail and relatively large wings. They are slightly smaller than wolves, and are too small to be ridden, though too large in order to use for the hunt. Though it was stated they were black in How to Speak Dragonese, they were green in the "Complete Book of Dragons: Guide To The Dragon Species".
Personality
Flashfangs are very sly, curious and untrainable dragons, that will often raid the sheep and deer herds of the vikings. Because of this, the tribes organise hunting parties every now and then, in order to keep their numbers to a limit.