In J.R.R. Tolkien’s The Hobbit, the dragon guarding the Lonely Mountain’s treasure is called Smaug.
Smaug takes possession of Erebor, the kingdom and treasure-hoard of the Dwarves, after attacking the mountain. His arrival drives the Dwarves into exile and leaves the nearby town of Dale ruined.
Bilbo Baggins enters the mountain during Thorin Oakenshield’s quest. He speaks with Smaug and discovers a bare patch in the dragon’s jeweled armor, information that later helps Bard the Bowman defeat him.
Smaug is one of Tolkien’s most recognizable dragons, but he is not the first or largest dragon in Tolkien’s wider mythology. Earlier tales describe Glaurung, while Ancalagon the Black appears in the history of the First Age. Those figures belong mainly to The Silmarillion and related writings.