The Seadragonus Giganticus Maximus is a type of dragon. These "preposterously huge" fire-breathing dragons are the size of a mountain, and are very dangerous; they kill for sport. When a Seadragon appears on the beach looking for a fight, Hiccup is the only one with a plan to save the tribe.
Seadragonus Giganticus Maximus are known to posses powers of telepathy and limited ability to see the future. They can breathe fire continuously for at least an entire night, which is powerful enough to dramatically alter landscapes. One Seadragon, Wodensfang, has demonstrated possession of retractable teeth.
The promise of a Seadragonus Giganticus Maximus is considered to be unbreakable, although in practice there is nothing to ensure it.
Seadragonus Giganticus Maximus are huge, blue, green, or purple, and covered in marine corals, anemones, and shells. However as it is discovered in How to Betray a Dragon's Hero, these dragons have wildly varying size. They start off extremely small, grow to impossibly huge size, and sometimes shrink down to the earlier extremely small size. It is unknown why the similarly aged Green Death and Wodensfang were so differently sized. Possibly dietary differences control it.
The Green Death/Merciless
- Main article: Green Death
The Green Death (or formerly known as Merciless) is the most well-known Seadragonus Giganticus Maximus. It claims, in the book, that it is an artist by eating other dragons, and is very proud of its size. In his youth, Merciless was the leader of the Dragon Rebellion, until Hiccup the First, using the Dragon Jewel, made him leave the Archipelago and never return, or else his fate--to be killed by a boy named Hiccup--would come to pass. A thousand years later, he returns to the Archipelago and is killed by the cunning of Hiccup the Third.
The Purple Death/Horror
- Main article: Purple Death
The Purple Death (also called The Purple Horror) is one of three Seadragonus Giganticus Maximus to appear in the first book of the How to Train Your Dragon book series. It is said to be larger than the Green Death, but is eventually killed by it.
- Main article: Furious
Furious was Hiccup the Second's dragon, sometimes referred to as his "brother". After Hiccup was killed, he was captured and used by the Berserks as an executioner, and was turned bitter and evil over the years. He currently leads the Dragon Rebellion, whose goal is to exterminate the human race (Hiccup Horrendous Haddock III in particular).
- Main article: Wodensfang
- Main article: Toothless
In How to Betray a Dragon's Hero, it is revealed that the Toothless from the books is also a Seadragonus Giganticus Maximus. He is Hiccup the Third's dragon, a tiny, stuttering, toothless Common-or-Garden--so everyone thought. In time, Toothless will grow to the size of a mountain, possibly gain teeth, and then shrink again in his old age. He is the "Best Thing" of the King's Lost Things, and is currently in the possession of Alvin the Treacherous.
- It is an official fact that all the Hiccups have had a Seadragonus Giganticus Maximus. Hiccup Horrendous Haddock I's was the Wodensfang, Hiccup Horrendous Haddock II's was Furious, and our Hiccup's was Toothless. All of them have stitches over their hearts.
- The Red Death was used to be categorized as a Seadragonus Giganticus Maximus; the movie counterpart of the Green Death until very recently, however, the 'queen of the dragons' is suddenly re-classified and is now a newly confirmed species of Stoker Class.