A dragon back from the golden age.
A dragon is a giant flying scaly beast that is capable of killing you in numerous ways that you didn't even think were possible. Also, they can breath fire. Ain't that great?! Yep, dragons came quite in handy for Fire Nation barbeques... until they all got killed off of course.
A long long long long long long long long long time ago, the dragons decided to share their wisdom with the people of the Fire Nation and taught them how to firebend. Of course, this turned out to be a huge fucking mistake. In their usual attempts to keep their evil image, the Fire Nation screwed the dragons over several thousand years later and killed them all off in a genocide. Talk about a happy ending.