Smaug was the dragon who guarded the treasure in Erebor in The Hobbit.
Smaug seized the Lonely Mountain and its vast hoard from the Dwarves of Erebor before the events of Tolkien’s novel. The Dwarf king Thrór had established the kingdom there, and Erebor became famous for its wealth and craftsmanship. Smaug’s attack drove the surviving Dwarves into exile.
Bilbo Baggins enters the mountain as part of Thorin Oakenshield’s company. He speaks with Smaug and discovers a bare patch in the dragon’s diamond-hard underside. That information is later used by Bard the Bowman, who kills Smaug with the Black Arrow during the attack on Lake-town.
Smaug is often compared with other Tolkien dragons, especially Glaurung and Ancalagon, but they belong to different stories and ages. Smaug is the principal dragon of The Hobbit and the last great dragon encountered in that narrative.