Winterfleshers are very dangerous dragons despite their small size: Roughly 2-4 inches or about the size of a human finger.
A single Winterflesher is relatively harmless, but a group can strip an animal down to bone in minutes. A group of Winterfleshers is referred to as a Shoal.
They did not align themselves with either side of the Dragon Rebellion.
Winterfleshers appear in Alvin's war bunker while Hiccup is being lowered through a trapdoor. The trapdoor leads to freezing water swarming with them, and Alvin and Excellinor the Witch are hoping that Hiccup will give up the location of his parents. However, the Winterfleshers don't touch Hiccup to Alvin's surprise. Alvin also kicks one angrily back into the sea after it lands on the floor flopping around.
Winterfleshers are mentioned in this reference book, with stats.