The dragons known to the Hairy Hooligan Tribe are grouped into one of seven "classes" based on their similar abilities or features that they posses (unlike modern taxonomy that is based on shared physical characteristics). Dragons can also be moved to another class if new discoveries are made that would better fit the dragon in another class. The seven classes are Stoker, Boulder, Tracker, Sharp, Tidal, Mystery, and Strike. As mentioned in the exclusive short, Book of Dragons, and in "We Are Family, Part 2", Gobber's great-great-great grandfather Bork the Bold was the first to categorize dragons into different classes.
Stoker Class dragons are hot-headed 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:
- Flame Whipper
- Green Death
- Monstrous Nightmare
- Night Terror
- Red Death
- Shadow Wing
- Silver Phantom
- Small Shadow
- Terrible Terror
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. According to Fishlegs in Rise of Berk, Boulder Class dragons aren't picky eaters. The dragon species included are:
- Catastrophic Quaken
- Crimson Goregutter
- Grapple Grounder
- Whispering Death (also Screaming Death variant)
The Tracker Class is a new class discovered during the events of Race to the Edge. Natural born hunters, Tracker Class dragons have a highly acute sense of smell or taste that enables them to effortlessly track down and find things. The dragon species included are:
- Common Rockstomper
- Deadly Nadder
Sharp Class dragons are vain and prideful, and they all possess sharp body parts. The dragon species included are:
- Devilish Dervish
- Egg Biter
- Grim Gnasher
- Speed Stinger
Tidal Class dragons live in or near the ocean, and most of them do not breathe fire, with even those who are capable of doing so only rarely using it. The dragons in this class are generally larger in size than most of the other classes. According to Hiccup, Tidal Class dragons don't react well to signs of aggression. The dragon species included are:
- Luminous Krayfin
- Purple Death
- Sand Wraith
- Tide Glider
Little is known about the Mystery Class dragons due to how stealthy and sneaky they are or how they work. Dragons in this class are generally more feared than those in the other dragon classes. The dragon species included are:
- Cavern Crasher
- Death Song
- Hideous Zippleback
- Smothering Smokebreath
- Sweet Death
- Sword Stealer
Strike Class dragons are characterized by their "blazing speed, vice-like jaw strength, and extreme intelligence," as well as pin-point accuracy, powerful attacks, explosive firepower and a unique ability to allow them to navigate in their respective environment/atmosphere. The dragon species included are:
- Light Fury
- Night Fury
- Night Light
- Songwing (Fishlegs' Dragon Cards)
- Snow Wraith
- Triple Stryke
- Woolly Howl
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. The currently unclassified dragons are an original dragon from the Arena Spectacular, the four remaining unnamed dragon species in How to Train Your Dragon 2, the unknown dragons on Hiccup's map in How to Train Your Dragon 2, the three unidentified dragons that appear in the Book of Dragons and one that is considered to be mythical.
Fear Class dragons are stealthy, sneaky, and often have multiple heads. The former dragons of this class are the Hideous Zippleback, the Flightmare and the Snaptrapper. This class was removed and replaced with the Tracker Class, most likely because Vikings no longer fear dragons. The dragons that used to be in this class were:
- 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 Red Death also used to be a Mystery Class dragon, until it was moved to the Stoker Class. The Deadly Nadder was a Sharp Class dragon, until it was moved to the Tracker Class.
- With twenty known species, the Mystery Class is currently the largest Dragon Class.
- With eight known species, the Tracker Class is currently the smallest Dragon Class.
- The Submaripper and the Hobgobbler are the only dragons confirmed to belong in two classes.