Flystorm

Flystorm is the Deadly Nadder that Mildew trained while on outcast Island.

In Riders of Berk
This dragon's first (and only) appearence was on the episode We are family Part II. While stranded on outcast island with no method to return to Berk, Hiccup taught Mildew how to train a Deadly Nadder so that they could fly home. This Nadder was Flystorm.

Personality
Flystorm was described in Rise of Berk as being the only dragon that actually likes Mildew.