Smaug is the dragon sleeping on the dwarves’ treasure in The Hobbit.
Smaug attacked the dwarven kingdom beneath the Lonely Mountain, or Erebor, long before Bilbo’s adventure. He drove the dwarves and their king, Thrór, from their home and took possession of their immense hoard of gold, jewels, and heirlooms. Smaug then slept on the treasure inside the mountain.
Bilbo enters the dragon’s lair as the company’s burglar and steals a golden cup. During their conversation, Bilbo notices a bare patch beneath Smaug’s left breast, where the dragon’s armour-like scales have a vulnerable gap. A thrush later tells Bard the Bowman about this weakness.
Smaug is often confused with other dragons in Tolkien’s mythology. Glaurung and Ancalagon belong chiefly to the legends of the First Age, while Chrysophylax is a dragon from Tolkien’s separate story “Farmer Giles of Ham.” Smaug is specifically the dragon of Erebor in The Hobbit.