Hideous Zippleback

The Hideous Zippleback is a large, two-headed dragon. It is sly and surprising, but certain noises can confuse it and cause it to entangle its heads. The dragon has two personalities and two minds, one in each head, and one often argues with the other head over what to do. This dragon reminds some vikings of bickering siblings when they aren't running away from it. However, its two heads also give it a deadly weapon that is unique from any other dragon species. One of the dragon's heads breathes a green, flammable gas while the other head uses an electrical spark in its mouth to ignite the gas, creating lethal explosions.

In the film, Hiccup learns from Toothless that most dragons hate eels enough to actually fear them and uses this against a Zippleback to scare it back into its den during dragon training. At the end of the movie, a Hideous Zippleback is flown by Ruffnut and Tuffnut. Each head seems to agree with one twin.

The official Website describes the Hideous Zippleback as follows: "Of all the dragons that inhabit our world, one of the most unusual and dangerous is the HIDEOUS ZIPPLEBACK. The Zippleback is one of the largest dragons you may ever encounter. It is unmistakable with its twin heads. The heads are distinct, with separate thoughts and different personalities. Like most dragons, Zipplebacks can fly, but their wings are small and they spend most of their time on the ground. Zipplebacks are solitary, preferring to hunt alone. They come out after sundown, prowling dark forests for victims. The attack of a Zippleback is like no other. Instead of breathing fire, a Zippleback makes explosions. One head BREATHES gas, and the other head LIGHTS it."

"This two-headed threat is the longest dragon and has the smallest wings and stubbiest legs. But the way it produces fire makes up for any physical drawbacks. While one head shoots out a flammable gas, the other ignites it with a spark -- allowing for attacks from long distances. “It is based more on a Siamese cat combined with a devious snakelike quality,” DeBlois says."

Trivia

 * Although the offical website states that the Hideous Zippleback is not effective in flight because of its small wings, during the young Vikings' counterattack against the Red Death in the movie, it is shown that the Zippleback is agile, fast, and effective at flying.
 * The Hideous Zippleback probably resembles a Hydra (a mythological dragon said to have two heads and four legs).
 * The Hideous Zippleback's two heads working together is based on the proverb, "Two heads are better than one".
 * It is shown in a training addition to How to Train your Dragon (film) that Hideous Zipplebacks (and other dragons) do have a sense of humor, such as when a Hideous Zippleback attacked Ruffnut and Tuffnut in the addition while the two twins fight, the two heads grin at each other as the twins continue to fight, while dangling from the Zippleback's mouths!