Seadragonus Giganticus Maximus

The Seadragonus Giganticus Maximus is a type of dragon. These "preposterously huge" fire-breathing Sea Dragons are twenty times the size of a blue whale, and are very dangerous -- they kill for sport. When a Sea Dragon appears on the beach looking for a fight, Hiccup is the only one with a plan to save the tribe.

Anatomy
Sea Dragons are blue with an orange underside. They have very primitive, dinosaur-like characteristics, with heads that resemble a tyrannosaur, narrow sharp teeth and the small frill of a triceratops. They have black spikes from the back of the neck to its tail, and wings with a single membrane. They also have gigantic legs with red spikes on them. Sea Dragons are the biggest dragon species ever seen, and they can crush a whole Viking house easily.

The Green Death
The Green Death is the most well-known Seadragonus Giganticus Maximus. In the film, it is known as the Red Death.

In the Books
In the first book, the Green Death arrives on long beach with a smaller Seadragonus Giganticus Maximus (which it eats later on). Hiccup is sent to meet with it and asks if it comes in peace or war (as he is the only one who can speak Dragonese). The dragon says he comes in peace but that he's still going to eat them all. The Green Death and Hiccup discuss Singing Suppers, but during the conversation the Green Death moves in to eat Hiccup. Just as this happens the Green Death falls asleep, allowing Hiccup to escape. With the help of the other Hooligan and Meathead boys and their dragons, Hiccup creates a plan to kill the Green Death with the help of a third Seadragonus Giganticus Maximus, the Purple Death, which they trick into helping them. In the following battle the Purple Death is killed but the Green Death is fatally wounded. In anger, the Green Death swallows Hiccup whole. Fortunately, Hiccup avoids being digested by grabbing and holding on to a Roman spear stuck in the dragon's throat. While inside the Green Death, Hiccup discovers the fire holes (a dragon's source of fire) and blocks them with his helmet. Due to this, the Green Death blows up when it tries to burn Stoick, Hiccup and Toothless. Its death makes Hiccup a hero in the eyes of his tribe.

In the Film
Sometime in production, the dragon's name was changed to the Red Death, although it is never referred to in the movie by name and it is blue (although, in the game, Stoick refers to the reason of the "Thor'sday Thursday" celebration as "the defeat of the Red Death"). It is the leader of the dragon nest, where it forces other dragons to bring it food to avoid being eaten themselves. When the Vikings try to destroy the nest, the dragon unleashes its fury upon the Vikings. When the Vikings try to retreat, the Red Death burns the Vikings' boats. When all seems hopeless, Hiccup and his friends arrive. During the fight, Hiccup comes up with a plan that will destroy the dragon. Hiccup makes Toothless annoy the dragon by spitting fireballs at it, causing it to unfold its wings and pursue Hiccup into the heavily-clouded sky. When the dragon starts to gain on them, Hiccup uses the cover of the clouds in order to hide from the dragon, and when the dragon tries to breathe fire on Hiccup, Toothless blasts fire into its mouth, causing the highly flammable gas to violently explodes. The Red Death then plummets back to earth and explodes upon impact.

Furious
In the latest book to date, another Seadragonus Giganticus Maximus, called Furious, is used as a form of execution by Berserks. The dragon flies off after being freed by Hiccup and saying it will lead the Second Great Dragon Revolution.

Trivia

 * While clearly meant to serve a similar role as the Green Death from the book, the antagonist, the Red Death has quite a few differences from the Green Death. For one, he doesn't appear to be a sea dragon and it lives in lava, and whereas the Green Death is almost a victim of fate, the Red Death is largely responsible for all the trouble caused in the movie.


 * In the Art of How to Train Your Dragon book from the film, the original designs and sketches all depicted it as a far more sea dragon inspired creature to the point of it being eel like and covered in coral. The final design, however, went with a combination of in-book traits and kept the aforementioned coral-like spikes.


 * This dragon is the largest seen in the film by a wide margin. It has a large club like tail and thick skull. An odd trait of the this dragon is it has six eyes, three on each side, and can fly despite being so big.


 * Its only weakness is confusion by noise, or pain to the eyes or mouth, but noise only affects it near its ear, and since the only way to get near it is by flying on a dragon, sound is ineffective.