The dragon who ruled the Lonely Mountain in J.R.R. Tolkien’s The Hobbit was Smaug.
Smaug seized Erebor, the Lonely Mountain, from the dwarves and drove them into exile. He then guarded the enormous treasure hoard inside the mountain. Thorin Oakenshield, heir to the former dwarf king, leads the company that attempts to reclaim the kingdom with Bilbo Baggins’s unexpected assistance.
Bilbo enters Smaug’s lair and converses with him using riddling language. During their exchange, Bilbo notices a bare patch in the dragon’s armored underside. The information eventually helps Bard the Bowman kill Smaug with a black arrow when the dragon attacks Lake-town.
Smaug is not Tolkien’s only dragon. Glaurung appears in the First Age story of Túrin Turambar, while Ancalagon the Black is associated with the War of Wrath. Smaug is specifically the dragon of The Hobbit and the last major obstacle before the Battle of the Five Armies.