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
- Monstrous Nightmare
- Night Terror
- Red Death
- Silver Phantom
- Terrible Terror
Notable dragons in the Stoker Class include Hookfang, Torch, Torch's Mother, Torch's Siblings, Fanghook, Gruff, the Snotlout's Fireworm Queen, the Solar Flare, the Red Death, Sharpshot, Iggy, Sneaky, Pain, Butt and Head, the Kick-off Terrors, Gothi's Pet, Valka's Mercy, Smidvarg and his pack, Girl Hookfang and her offspring, Hookfire, Fangmaster and Hookblazefang, Hookfang's Nemesis, Firescrapes, Cagecruncher, Darkvarg and his pack of albino Night Terrors, Nightwatch, Sven's Nightmare, Trap-phoomerang, Flashfright, Scorch and his allies, Krogan's Singetail, the Ambered Singetail, Deathlout, Flamelout, Heatlout, Hotlout, Stoick, Sacred Terror, Primal Hobblegrunt, Seedling Night Terror, Garden Dragon and Royal Fireworm.
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
- Grapple Grounder
- Whispering Death
Notable dragons in the Boulder Class include Meatlug, Toothless' Nemesis, Groundsplitter, Book Wyrm, Grump, Lump, Clawlifter, Tormentor, Basket Case, Tuffnut Junior, Junior Tuffnut and Junior Tuffnut Junior, Gothi's Gronckle, Shattermaster, Trailtwister, Guslout, Tuffnut's Death Ride, Spikeback, Furnace, Exiled Gronckle, Frozen Groncicle, Primal Snafflefang, Irontooth, Champion Catastrophic Quaken, the Great Protector, the New Protector, Rushing Death, Magmadon and Yawnckle.
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:
Notable dragons in the Tracker Class include Stormfly, Flystorm, Skullcrusher, Kingstail, Scardian, Sailback, Rumpus, Gustnudger, Pestbud, Sneezlehunch, Bork Week Nadders, Edge Nadder, Ambered Nadder, Steeltrap, Seedling Rumblehorn, Champion Windgnasher, Bonesnarl and Muddie.
Sharp Class dragons are vain and prideful, and they all possess sharp body parts. The dragons included are:
- Devilish Dervish
- Egg Biter
- Grim Gnasher
- Speed Stinger
Notable dragons in the Sharp Class include Cloudjumper, Thump, Stokehead, the Lead Stinger and his pack, Windshear, Soaring Sidekick, Speedy, Winged Warden, Snaggletooth, Thornshade, Axewing, Egg Blossom, Chompers, Puddlemuck, Hackagift, Slushclaws, Icebreaker, Reignstorm, Fighter Razorwhip, Wingnut, Thorstopian, Quiverpain, Brute Timberjack, Primal Thornridge, Dammen's Timberjack, Taxi, Seedling Speed Stinger and Nipper.
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 stated that Tidal Class dragons don't react well to signs of aggression. The dragons included are:
- Luminous Krayfin
- Purple Death
- Sand Wraith
- Tide Glider
Notable dragons in the Tidal Class include Thornado, Bing, Bam, Boom, Scauldy, Whirlwing, Mildew's Misery, Valka's Bewilderbeast, Drago's Bewilderbeast, Berserker Bewilderbeast, Thunderfish, Earsplitter, Lil Lullaby, Bonecrusher's Conquest, Covecharger, Bolt & Float, Valka's Three Seashockers, Shipsbane, Seedling Sand Wraith, Sweet Wraith, Desert Wraith, Deepfreeze, Grazefall, Rogue Submaripper and Skykarver.
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
Notable dragons in the Mystery Class include Barf and Belch, Gobber's Nemesis, Hofferson's Bane, Bandit, Whip and Lash, Edgewing, Viggo's Flightmare, Big Buff, Outsnapper, Muddlehunt, Phantom, Sparks and Spew, Trancemare, Dustbrawler, Coldsnap, Tuffwing, Garffiljorg, Garff's Adoptive Father, Slimeslinger, Fishlegs' Dramillion, Skystreaker, Firecomb Crasher, the Titan Wing Dramillion, the Titan Wing Zippleback, Incognito, Seedling Sweet Death, Brute Boneknapper, Brute Snaptrapper, Death & Chocolates, Rattling Smokebreath, Gritpicker, Gresskarmor, Gobstinker, Snogglesong, Masked Sweet Death, Boneshedder, Dammen's Boneknapper, Slimeball and Springwing.
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 dragons in this class are some of the rarest of all dragons, some are close to extinction. They are also the most difficult to train but the most loyal once they have been. They are also generally among the most powerful dragons. The dragons included are:
Notable dragons in the Strike Class include Toothless, the Unnamed Light Fury, the Frozen Skrill, Gothi's Frostfright, Sleuther, Viggo's Skrill, Wise Wind, Frostfright's Big Bro, Frostfright's Lil Sis, Icebane, Skrill Chiller, Mildew's Strain, Hunterbolt, Brute Skrill, Champion Triple Stryke, Snoggletog Wraith, Lavender Skrill and Eclipser.
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, three new dragon species from How to Train Your Dragon: The Hidden World, the three unidentified dragons that appear in the Book of Dragons, the flightless dragon species that appears in Dragonvine, the two dragon species from Race to the Edge, one dragon species that appears in Dreamworks Dragons: Dawn of New Riders, and one that is considered to be mythical.
- Crimson Goregutter
- Shadow Wing
- Small Shadow
- Unknown Dragons
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 the Vikings no longer fear dragons. The dragons that used to be in this class were:
- 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 symbol of the Strike Class was changed by the Hooligan tribe 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 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 eighteen known species, the Mystery Class is currently the largest Dragon Class.
- With six known species, the Strike Class is currently the smallest Dragon Class.
- The Deadly Nadder and the Hideous Zippleback respectively symbolize the Sharp Class and the Fear Class, despite no longer being in them.
- The Submaripper is the only dragon to belong in two classes, in its case, the Tidal and Tracker Classes. This is due to its abilities fitting in with both classes of dragons.