The dragon who guards the Lonely Mountain in J.R.R. Tolkien’s The Hobbit is Smaug.
Smaug occupies the mountain after driving the Dwarves of Erebor from their kingdom and seizing their treasure. Bilbo Baggins enters the mountain as part of Thorin Oakenshield’s company and speaks with the dragon, learning that Smaug has a vulnerable patch on his underside.
The information ultimately reaches Lake-town, where Bard kills Smaug with a black arrow. Smaug is not the same dragon as Glaurung, who appears in Tolkien’s earlier legendarium story about Túrin Turambar, or Ancalagon, a dragon from the First Age. Those names belong to different periods of Middle-earth’s history.