Red Death

The Red Death is one of the Seadragonus Giganticus Maximus species. It appears in How to Train Your Dragon. The gender is possibly female, as it is known as the queen of the Mountain Nest of the dragons. And since its actually called the green death in the soundtrack (They obviously changed it. It does not actively hunt but it has its food delivered to it by other dragons in the nest. It will eat any dragon that does not bring sufficient food; when a Gronckle brought only a single fish, the Red Death devoured it moments later.

After a hoard of Vikings led by Stoick attacked the nest, the Red Death burst out of the mountain and attacked them. It is eventually killed by Hiccup and Toothless, who damaged its wings and tricked it into diving downward, then used Toothless' fire to ignite its internal gasses, causing it to blow up.

Dragons of the Red Death
The Dragons of the Red Death are a group of dragons that are enslaved and forced to serve a injurious parasitoid called the Red Death. All or most dragons in the movie appear to serve the Red Death. The Red Death has been compared to a queen bee or a brood parasite. It is the leader and all others serve it apparently out of fear of being eaten, although it is unknown for certain how it became the leader of the other dragons (It is possible that it invaded their home and proclaimed itself as such or perhaps some  kind of mind control as a Seadragunus giganticus maximus in the book was able to create a swarm f them under his control to start a rebellion

Hiccup and Astrid atop Toothless discovered this, when Toothless followed a group of dragons to the volcano where the Dragons' Nest was contained within.

Stoick the Vast got this information out of his son, and soon led a party to where Hiccup had explained, using a restrained Toothless as their guide.

Hiccup helps his friends tame their own dragons to rescue Toothless from the Hairy Hooligans who captured the dragon in hopes of being lead to the Red Death. Hiccup allies his friends with these dragons: Astrid and her Deadly Nadder, Snotlout and his Monstrous Nightmare, Fishlegs and his Gronckle, and Ruffnut and Tuffnut and their Hideous Zippleback. Hiccup then shares a ride with Astrid, and they part for the Red Death's lair where they'll find Toothless and the Hairy Hooligans.

The Hooligans had opened fire on the side of the volcano, freeing the dragons inside and waking the Red Death. The dragons had gone, flying away, in fear of the wrath of the Red Death. The Hooligans, though, were not aware of this until it attacked them.

Hiccup and Stoick later help free Toothless from his bonds, and together, Hiccup and Toothless bombard the Red Death with their friends. The duo lead the Red Death to the sky, into the clouds. The Red Death had lost track of them in the clouds, as they were using them as cover. Toothless shot the Red Death in a sneak attack, and then disappeared into the clouds; he repeated this multiple times, until the Red Death was enraged, and set the surrounding area in fire. Toothless' synthetic tail caught fire, and they had to hurry their plan.

Hiccup drove Toothless downward, and the Red Death followed close behind. Toothless dove as fast as he could manage, though the Red Death always strayed closer. Just before the Red Death was about to fire on them, Hiccup turned his dragon and Toothless shot a bolt of thunder into the Red Death's mouth, overheating it. With the straying embers off Toothless' tail, the wings of the Red Death caught fire, and burned holes into the skin. The Red Death lost control and crashed into the shoreline, exploding to its death on impact.

With the Red Death now gone, the dragons are now free and no longer have to raid the island of Berk of their food anymore, and the Hooligans were taught to live with them in peace. However there still the threat of another seadragonus giganticus maximus popping up

Trivia

 * The Red Death could be a reference to the male from the film Reign of Fire as they both atr the largest dragons, are giants, resort to cannabolism on smaller dragons, and act as the "Queen."