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
"It has six eyes, four legs and has NO weak spot!"

- Fishlegs, on the Red Death

Sea Dragons are blue with an orange underside. They have very primitive, dinosaur-like characteristics, with heads that resemble a tyrannosaurus, narrow sharp teeth, small frills similar to those of triceratops, thick pillar-like legs like those of sauropods, and a clubbed tail similar to that of ankylosaurus. They have red spikes from the back of the neck to the 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 (though the soundtrack lists "Battling the Green Death" as the background music to the final battle with the enormous dragon). It claims, in the book, that it is an artist by eating other dragons, and is very proud of its size.

The Purple Death
The Purple Death 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 as large as the Green Death and is eventually killed by it.

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, 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: it doesn't appear to be a sea dragon and instead seems to be more dinosaur-like, 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 for what is believed that the Green Death is a Major antagonist but not the main, it is believed in the book that the main villains are: Snotlout and Fireworm and in the movie Stoick is the true Villain because of his war-mongering against dragons.
 * 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 and having small, beat-up wings.
 * Its only weakness would be confusion by noise, but noise only affects it near its ear; since the only way to get near it is by flying on a dragon, sound is, in fact, ineffective due to its effects on the dragon being ridden as well.
 * In the movie it's shown to have a more dinosaur-like head and mouth while in the book his head and mouth resemble more a classic european dragon.