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.

Anatomy
"Uhhh.... This thing doesn't have a blind spot!" - Fishlegs, on the Red Death

Seadragons 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 and five claws on the ends. Unusually for a dragon, they possess three sets of eyes, three on each side of the head. Seadragons are the biggest dragon species ever seen, around the same size of titanosaur sauropods, and they can crush a whole Viking house easily.

The Green Death/Merciless
The Green Death (or formerly known as Merciless) is the most well-known Seadragonus Giganticus Maximus. In the film adaption, 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/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
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 its head and mouth resemble more of a classic European dragon.
 * 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 account parallels the Red Death.
 * According to developer commentary the final fight with the Red Death was modified in it's last scenes. After Hiccup and Toothless send a bolt of fire into it's stomach and it begins to explode, they fly down it's spine and away to safety. In this scene the dragon's size was tripled to make the scene more dramatic.
 * The Red Death could be a reference to the male from the film Reign of Fire as they both are the largest dragons, resort to cannabolism on smaller dragons, and act as the "Queen."