Smaug was the dragon who guarded the treasure of the Lonely Mountain in J.R.R. Tolkien’s The Hobbit.
Smaug drove the Dwarves from Erebor and took possession of their vast hoard. He then slept beneath the mountain until Bilbo Baggins entered the lair as part of Thorin Oakenshield’s quest. Bilbo’s conversation with Smaug reveals that the dragon has a vulnerable, unarmoured patch on his underside.
That information reaches Bard of Lake-town through the thrush that overhears Bilbo’s encounter. Bard uses a black arrow to kill Smaug when the dragon attacks Lake-town. Smaug is not the same dragon as Glaurung, the first dragon of Morgoth, or Ancalagon the Black, who appears in the First Age. Those dragons belong to different periods and stories in Tolkien’s legendarium.