Seadragonus Giganticus Maximus

The Seadragonus Giganticus Maximus is a type of dragon. These "preposterously huge" fire-breathing dragons are twenty times the size of a T-Rex, 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.

Physical Appearance
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 start of small, grow to their full size, then shrink back to the size of a hunting dragon.

The Green Death/Merciless


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
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.

Furious


Furious was Hiccup the Second's dragon, sometimes reffered to as his "brother". After Hiccup was killed, he was 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).

The Wodensfang


It is revealed in How to Betray a Dragon's Hero that the Wodesnfang is a Seadragonus Giganticus Maximus. He was the dragon of Hiccup the First. He stole the Dragon Jewel and gave it to Hiccup I, stopping the first dragon rebellion. When Hiccup III came around, he gave Hiccup III the Crown in the Fire Pit. Later, he makes a deal with the Dragon Furious that he will betray whoever becomes King of the Wilderwest to the Rebellion.

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.

Trivia

 * According to a collection of books by Claudius Aelianus (Greek: Κλαύδιος Αιλιανός) called On Animals, Ethiopia was inhabited by a species of dragon that hunted elephants. It could grow to a length of 180 feet and had a lifespan rivaling that of the most enduring of animals. This may have been a minor inspiration for Seadragonus Giganticus Maximus.


 * 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. Coincidence, or what?


 * 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.