Hideous Zippleback (How to Train Your Dragon)

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.

Behavior
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. The Hideous Zippleback's spine has many bright red lobes that run down it. Occasionally the Zippleback will us these to hold it necks together, making it less intimidating. They have serrated fangs which they use to inject venom for predigestion. Its top teeth are much sharper and are all the same size, while its bottom teeth are very long and stick up above its jaw line.

Movie
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.

Description
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 & 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."