The Hideous Zippleback belongs to the Mystery Class of dragons in How to Train Your Dragon.
The species is distinctive because it has two heads, with each head producing a different gas. One head generates a flammable gas and the other creates a spark, allowing the dragon to ignite the mixture. In the films, Ruffnut and Tuffnut ride a Hideous Zippleback named Barf and Belch.
The dragon-class system groups species by shared abilities and traits. Mystery Class dragons are generally associated with unusual, deceptive, or hard-to-understand characteristics. The exact classification terminology is part of the franchise’s fictional dragon lore.
The Hideous Zippleback is often confused with other two-headed or unusual dragons. Its defining feature is not simply having two heads, but having coordinated gas-and-spark abilities that create its signature fire attack.