In J.R.R. Tolkien’s legendarium, Smaug is the dragon who guards the Lonely Mountain’s treasure.
Smaug is the principal dragon in The Hobbit. He attacks the Dwarf kingdom of Erebor, also called the Lonely Mountain, and drives its inhabitants into exile. After taking the mountain’s vast hoard, he sleeps on the treasure for many years while the dispossessed dwarves live elsewhere.
Bilbo Baggins enters the mountain with Thorin Oakenshield’s company and speaks with Smaug. During their conversation, Bilbo notices a bare patch in the dragon’s otherwise protected underside. The information eventually reaches Bard the Bowman, who uses a black arrow to kill Smaug during the attack on Lake-town.
Tolkien created other famous dragons, so names can be confused. Glaurung belongs chiefly to The Silmarillion’s story of Túrin Turambar, while Ancalagon is a vast dragon from the First Age. Smaug is the dragon specifically associated with The Hobbit and Erebor.