Bullheart was Stoick the Vast's riding dragon. Bullheart betrayed Stoick in How to Steal a Dragon's Sword by attacking him. This is most likely because Alvin the Treacherous (who became the new King of the Wilderwest in waiting) spoke of chaining up the dragons, angering him. This could also be because of Stoick's mistreatment of his dragons. It is unknown what species he was.