Sozin's dragon

Sozin's dragon was a blue dragon that Fire Lord Sozin used as a mount and his animal guide. As vicious as his master, Sozin's dragon was able to fly over great distances at very fast speeds. Sozin was the only Fire Lord known to have a dragon.

History
Not much is known about the dragon's history or when Sozin got it, but Sozin used his dragon later in life to cross the hundred miles that separated the Fire Nation Capital and Roku's island in order to help his old friend battle the erupting volcanoes that threatened to engulf it in fire. When Roku was fainting from the poisonous volcanic gases, Sozin's dragon swooped in and saved only his master, while the pyroclastic flow swept over the Avatar and killed him.

Trivia

 * Sozin's dragon, together with Roku's dragon, Fang, represented the comparison of red and blue dragons; Fang was red and represented good, whereas Sozin's dragon was blue and represented evil.
 * Despite the fact that Sozin owned a dragon, he started the tradition of hunting them.