The dragons known to the Hairy Hooligan Tribe are grouped into one of seven classes based on their characteristics and abilities. The seven classes are Stoker, Boulder, Tracker, Sharp, Tidal, Mystery, and Strike. As mentioned in the exclusive short, Book of Dragons, and in episode 20 of Dragons: Riders of Berk, Gobber's great-great-great grandfather Bork the Bold was the first to categorize dragons into different classes.
Stoker Class dragons are hotheaded fire breathers. This class includes dragons that can set themselves on fire, as well as most of the smaller dragon species. A Stoker Class dragon losing its fire is a sign that it is going to die, although its flames can be reignited by a Fireworm Queen's venom or by eating their Flame Gel combs. The dragon species included are:
- Monstrous Nightmare
- Terrible Terror
- Red Death
- Night Terror
Notable dragons in the Stoker class include Hookfang, Torch, Torch's Mother, Torch's Siblings, Fanghook, Gruff, the Monstrous Nightmare that Astrid trained, the Wild Fireworm Queen, the Red Death, Sharpshot, Iggy, Sneaky, Pain, Butt and Head, the Kick-off Terrors, Gothi's Pet, Smidvarg and his pack, Girl Hookfang and her offsprings, Hookfire, Fangmaster and Hookblazefang, and the Titan Wing Monstrous Nightmare.
Boulder Class dragons are tough and are associated with the earth. They can eat rocks, and although they have small wings relative to their body size, they are able to fly as fast and as high as most other dragons can. The dragon species included are:
- Whispering Death
- Grapple Grounder
- Catastrophic Quaken
Notable dragons in the Boulder class include Meatlug, Toothless' rival, Groundsplitter, the Screaming Death, Grump, Lump, Basket Case, Tuffnut Junior, Junior Tuffnut and Junior Tuffnut Junior and Tuffnut's Death Ride.
The Tracker Class is a new class discovered in Race to the Edge. The symbol of this class reuses the Fear Class symbol, despite the Hideous Zippleback not belonging to this Class. Natural born hunters, Tracker Class dragons have a highly acute sense of smell or taste that enables them to track down and find things. The dragon species included are:
Sharp Class dragons are vain and prideful, and they all possess sharp body parts. The dragons included are:
- Speed Stinger
- Devilish Dervish
Tidal Class dragons live in or near the ocean, and unlike most dragons, they do not breathe fire as their ranged attacks, and although very few can, they rarely do use them. The dragons in this class are generally larger in size than most of the other classes. Hiccup also said that a Tidal class dragon cannot stay out of the water for a long time, but this is false for some of these aquatic dragons. The dragons included are:
- Sand Wraith
- Tide Glider
Little is known about the Mystery Class dragons due to how stealthy and sneaky they are. Dragons in this class are generally more feared than those in the other dragon classes. The dragon species included are:
- Hideous Zippleback
- Smothering Smokebreath
- Sweet Death
- Sword Stealer
- Death Song
Strike Class dragons are characterized by their "blazing speed, vice-like jaw strength, and extreme intelligence," as well as pin-point accuracy, powerful attacks and explosive firepower. The dragons in this class are the rarest of all, close to extinction. They are also the most difficult to train but the most loyal once they have been trained. The dragons included are:
Some dragons have not yet been officially classified into any class and may be sorted in the future. The reasons for this lack of information varies. Firstly, it is simply because the dragon is too new or had only made very few appearances in media. Usually, this happens when the dragon was created by a company other than Dreamworks. The currently unclassified dragons include the three original dragons from the Arena Spectacular, the unknown dragons in How to Train Your Dragon 2, three unidentified dragons that appear in the Book of Dragons short, a currently unclassified dragon from the comic books, and a currently unclassified new dragon who will appear in Race to the Edge.
- Egg Biter
- Unknown Dragons
- Cavern Crasher
- Seadragonus Giganticus Maximus
- Unidentified Dragon (Book of Dragons)
- Unidentified Dragon Two (Book of Dragons)
- Unidentified Dragon Three (Book of Dragons)
Fear Class dragons are stealthy, sneaky, and often have multiple heads. The former dragons of this class are the Hideous Zippleback and the Snaptrapper. This class was removed and replaced with the Tracker class, most likely because the Vikings no longer fear dragons any more.
- Since the Vikings didn't know what a Night Fury looked like until the taming of Toothless, the Strike Class should logically be symbolized by the Skrill, or another Strike Class dragon. It is possible that the Strike class's symbol was changed by the Hooligans to have a new symbol after the discovery and taming of Toothless.
- It is possible that the Mystery Class is a sort of "temporary" class where dragons are put when little is known about them, and then when they are more thoroughly understood they are moved to another class. For instance, the Night Fury could have been a Mystery Class dragon before the events of the first movie, and then moved to the Strike Class once it was studied and trained by Hiccup. However, due to all of the Fear Class dragons being moved into the Mystery Class, this may no longer be true.
- It has been shown that the dragons are occasionally reclassified. All Fear Class dragons were moved into the Mystery Class. The Flightmare was a Strike Class dragon, until it was moved to the Mystery Class. The Red Death also used to be a Mystery Class dragon, until it was moved to the Stoker Class. The Snafflefang used to be a Stoker Class dragon, until it was moved to the Boulder Class. The Deadly Nadder was a Sharp Class dragon, until it was moved to the Tracker Class. The Hobblegrunt used to be a Boulder Class dragon, until it was moved to the Stoker Class.
- With twelve known species, the Sharp Class is currently the largest Dragon Class.
- With four known species in each Class, the Tracker Class and the Strike Class are currently the smallest Dragon Classes.
- The Deadly Nadder, the Hideous Zippleback and the Grapple Grounder symbolize the Sharp Class, Tracker Class and the fanmade Unknown Class, despite no longer being in them.