The Fire Dragon is a legendary dragon that can only be hatched by the lava in a volcano. This dragon first 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. A Fire Dragon hatched in How to Twist a Dragon's Tale because Stoick stole the egg from the volcano, and it erupted, but straight afterwards, Toothless dropped it into the volcano, causing it to hatch.
Once they hatch, they feast on Exterminator Dragons. The Vikings were safe at the end of How to Twist a Dragon's Tale because the Fire Dragon eats only Exterminators. There has been a rumour that Fire Dragons eat Night Furies.