Tolkien’s fictional dragon who attacks the Lonely Mountain is Smaug.
Smaug appears in The Hobbit as the dragon who drove the Dwarves from Erebor and seized their treasure. Thorin Oakenshield’s company travels to the mountain to reclaim their homeland, with Bilbo Baggins serving as the burglar who enters Smaug’s lair.
Bilbo discovers that Smaug has a vulnerable patch on the underside of his left breast. After Smaug flies to attack Lake-town, Bard the Bowman uses the Black Arrow to kill him. Smaug’s death changes the political balance of the region and helps trigger the Battle of the Five Armies.
Tolkien created other dragons, including Glaurung and Ancalagon, but they belong mainly to the earlier histories told in The Silmarillion. Smaug is the dragon most directly associated with The Hobbit and the Lonely Mountain.