Flashfangs are thin, black dragons with long fangs, a hooked tail and relatively large wings. They are slightly smaller than wolves, and therefore 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 appear greenish in The Incomplete Book of Dragons, despite being called "black panther-like creatures" within its pages.
In Book 3 they are also described as being "armored", though what exactly the armaments are is not made clear.
|“||For creeping into the arena came twenty heavily armored black Flashfangs, their razor-sharp teeth glistening in the sunshine, their talons making slashing patterns in the dust.||”|
|— Book 3|
Flashfangs are one of the dragon species that can speak Dragonese.
Flashfangs are not able to swim, as seen in How to Speak Dragonese in the Roman Arena.
|“||What about the F-F-Flashfangs?" asked Toothless from somewhere inside Hiccup's shirt.
"The water will kill the Flashfangs," said Hiccup. "Flashfangs can't swim ...
|— Book 3|
Despite having a Disobedience score of "6", Flashfangs are said to be un-trainable. They are very sly, curious dragons, that will often raid the sheep and deer herds of the Vikings. Because of this, the tribes organize hunting parties every now and then, in order to keep their numbers to a limit.
They did not select a side in the Second Dragon War.
The Flashfangs were part of the circus show taking place at the Roman's amphitheater in Fort Sinister. 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 in preparation for Hiccup's, Camicazi's, and Fishlegs's portion of the entertainment, effectively killing off the Flashfangs.
Flashfangs are mentioned in this reference book, as well as appearing in a picture.