Though no depiction exists for these dragons, they are described as being "about the size of a dog". Hiccup notes (despite having memory loss) that Fernwinders are:
|“||Wingless, medium-sized dragons that carve tunnels through the forests of ferns in the Archipelago so they can charge across the islands at surprising speeds.||”|
|— How to Fight a Dragon's Fury|
Their movement is described as "charging through the undergrowth like terrified little rhinoceroses". They also have "spiny fins", which Hiccup utilized by grabbing onto them to be dragged rapidly across the island.
These dragons live on the Isle of Tomorrow as well as other islands in the Barbaric Archipelago, and seem to be unaligned during the Dragon Rebellion. If Fernwinders were to be assigned a class, they would most likely be Bog Dragons, due to their habitat.
Fernwinders are secretive dragons that build tunnels through the boggy vegetation on the Isle of Tomorrow. The tunnels serve to protect the Fernwinders from detection by aerial predators, as well as an easy way to move about the bogs.
In this final Book, Fernwinders inhabit the bogs and heather and bracken of the Isle of Tomorrow. Hiccup, fighting memory loss from his ordeal at sea, manages to get past the Druid Guardian and is dropped by the Dragon Guardians of Tomorrow. He survives and seeks to move quickly across the island, so hitches a ride by grabbing ahold of a panicking Fernwinder.