Smaug the Impenetrable (T.A. ~2770 - 2941) was the greatest fire-breathing dragon of the Third Age. His origins are unknown, but he appeared on the scene during the Sack of Erebor in 2770 when he claimed the Lonely Mountain and drove the Dwarves from Erebor. There he remained for nearly two hundred years until Thorin and Co. returned to reclaim the Mountain and its treasure.
Once Bilbo Baggins refers to himself as “Barrel-rider,” Smaug comes to believe that the Men of Lake-town—who deal with many barrels in their workings with the Elves of Mirkwood—are involved in the quest to reclaim Erebor. He then desolates Lake-town in a terrible attack. He is slain by Bard the Bowman, and the reign of Smaug is ended.