Dragonmice are, as nanodragons are, extremely tiny — the size of insects. They have a large spaded tail, two pair of legs, and a pair of legs. Their notable feature is their long, rabbit-like ears, which appear to nearly be as long as their bodies (not including the tail).
Drosy-Tipped Dragonmice are extremely active during their waking hours, appearing as only a blur as they catch and eat their prey (presumably bugs). However, a Dragonmouse's waking hours are only 15 minutes. For the rest of a 24 hour period they are fast asleep.
Like all Nanodragons, they remained uninvolved during the Dragon Rebellion.
Dragonmice are illustrated and named in this reference book, in the section discussing Nanodragons.
In this final Book of the series, the Dragonmouse makes a small appearance along with other Nanodragons, when Ziggerastica appears with his Nanodragon kingdom to help Hiccup obtain the crown of the Wilderwest.