The dragon Bilbo Baggins confronts in J.R.R. Tolkien’s The Hobbit is Smaug.
Smaug is a powerful fire-drake who has occupied the Lonely Mountain for generations. He drove the Dwarves of Erebor from their kingdom and claimed their treasure, including the Arkenstone. Thorin Oakenshield leads the company that hopes to reclaim the mountain, with Bilbo serving as burglar.
Bilbo first speaks with Smaug while hidden inside the mountain. Their conversation reveals Bilbo’s quick thinking and Smaug’s pride, but Bilbo inadvertently gives the dragon information about Lake-town and the expedition. Smaug later attacks Lake-town, where Bard the Bowman kills him with the Black Arrow.
Glaurung, Ancalagon, and Scatha are also dragons in Tolkien’s legendarium, but they belong to other stories and ages. Smaug is specifically the dragon of The Hobbit and is associated with Erebor and the Lonely Mountain.