Unnamed Islands

There are Unnamed Islands or locations mentioned in various media across the How to Train Your Dragon Franchise.

In Dragon Down (Comic)
In this comic, Hookfang is sent to a cave while he is shedding his flammable scales. Feeling dejected, he flies away to a volcanic island that is not named in the story.

The island has a central, active volcano with hot caverns inside. The combination of stressed Stoker dragon and active volcano means the volcano could erupt.

Rise of Berk
In the Game, Rise of Berk, there is a location that is searchable with Toothless (Franchise) that has no name.