The Fire Dragon is an ultra-rare dragon that can only be hatched by the eruption of a volcano. This dragon appears in How to Twist a Dragon's Tale. The thing that makes them so rare is that while they can only be hatched by an erupting volcano, the chemicals in the egg stop the volcano from erupting. Once they hatch, they feast exclusively on Exterminator Dragons (and the odd human).
Fire Dragons are extremely huge and have scales lit on fire, to the point that the dragon seems entirely made of flames.
- How to Twist a Dragon's Tale (Book 5), Cressida Cowell. 2007