In 2014, Cressida Cowell released The Incomplete Book of Dragons in the UK and The Complete Book of Dragons in the US. Both are identical except for the In-. These books detail many dragon species seen in the How to Train Your Dragon Books, as well as some that aren't seen in the series. Also introduced in these books are a classification system for the Book dragon species. Any classification is not really mentioned in the actual series. It is possible that the author introduced a classification system not only to organize the plethora of species, but also because classification is very popular in the How to Train Your Dragon Franchise.
The author appears, in large part, to base species classification on habitat, and a minor extent on ability and size. The Classes for Book dragons are: Cave Dragon, Tree Dragon, Bog Dragon, Sky Dragon, Mountain Dragon, Nanodragon, and Sea Dragon. There are also a large number of dragon species for whom a Class has not been specified and remain unknown.
Cave Dragons are Dragons that spend most or all of their lives underground, especially in tunnels and caves that riddle the Barbaric Archipelago. A few species, however, may be classified due to ability, such as the Driller-Dragon. Many species are described as primitive. Cave Dragons are not to be confused with dragons which hibernate for the winter in caves.
|Burrowing Slitherfang (presumed)||Gronckle||Skullion|
|Eight-Legged Nadder||Monstrous Strangulator||Stickyworm|
|Electricsticky||Octodangle||Unknown Cave Dragon|
Tree Dragons are the species that live in and around the dense wild woods of the Barbaric Archipelago. For the most part they don't think much either way of humans, though a few are utilized by Vikings for riding such as the Red Tiger or other purposes like the Vampire Spydragon. A few species are opposed to humanity and join the Dragon Rebellion, like the Breathquenchers and Poison Darters. The species in this Class are:
|Breathquencher||Giant Bee-Eater||Shortwing Squirrelserpent|
|Cuckoo Dragon||Poison Darter||Vampire Spydragon|
Bog Dragons have adapted to the marshes, bogs and wetlands that can be found all over the Archipelago, the Isle of Berk included. Bog Dragons are abundant, with many species tamed and used by the Vikings.
Dragons of this Class are:
Nanodragons are extremely common and very small - they are the "insects" of the dragon world. They can be very stupid, but also wickedly intelligent. They are ruled by one dragon, which is always from the Emperor Beetleboog Bolderbug species. The Bolderbugs can live for several decades, but most nanodragon species are very short-lived.
The Nanodragon species are:
|Centidile||Emperor Beetleboog Bolderbug||Plankenteenie||Unknown Nanodragon|
|Electricsquirm||Long-Eared Flutterfire||Tiddly-Nip Tick-Botherer|
Mountain Dragons have members who are behaviorally at opposite extremes - some are the most aggressive and wildest of all species, while others are the 'Cuties' of the dragon world, used as lap-dragons. Some of the more violent species despise Vikings, and were the first to join the Dragon Rebellion against humans. Many species are very prized by Vikings, though, because of various abilities such as tracking, hunting, spying, electricity, or camouflage.
The Mountain Dragon Species are:
|Bullguard Slavedragon||Hellsteether (presumed)||Sniffer Dragon|
|Bullpuff (presumed)||Hogfly||Snub-Nosed Hellsteether|
Sky Dragons are the most agile and strongest fliers of the dragon world. They are often the species used as riding dragons by Vikings (but not exclusively). Many species allayed themselves with the humans during the Dragon Rebellion, and are often very loyal to their riders. Some species, such as the Silver Phantom are referred to as "Air Dragons". Perhaps this was an alternate name for this classification.
Species in this Class are:
|Rocket Ripper||White Dragon (presumed)|
|Devilish Dervish||Silver Phantom||Zebramount|
Most Sea Dragons are huge and ferocious monsters, mysterious as the seas they terrorize. There are some exceptions, such as the Golphin, which is portrayed as a smaller, friendly dragon. Dragons who have a connection with ice and snow are also in this Class, so broadly this Class contains dragons associated with water.
- "A REAL Sea Dragon is fifty times as big as that little creature. A real Sea Dragon from the bottom of the ocean can swallow ten large Viking ships in one gulp and not even notice. A real Sea Dragon is a cruel, careless mystery like the mighty ocean itself, one moment as calm as a scallop, the next raging like an octopus."
- ―Old Wrinkly in Book 1
Dragons without a Class
There are a large number of dragons mentioned in the Books that the author did not assign a class in The Incomplete Book of Dragons. They are as follows:
- Despite being Mountain Dragons, the Dragon List at the end of the book states that the Deadly Shadow, Triple-Header Rageblast and Horror are Sky Dragons and Sea Dragons, despite saying otherwise in earlier chapters.
- Seeing how they almost look alike, and have the same stats and description, the Deadly Nadder and the Eight-Legged Nadder may be the same species.