The dragon who occupies the Lonely Mountain in The Hobbit is Smaug.
Smaug is the chief dragon and central antagonist of J. R. R. Tolkien’s novel. He attacks Erebor, also called the Lonely Mountain, drives out the Dwarves, destroys the nearby town of Dale, and settles on the vast treasure hoard beneath the mountain. His presence is the reason Thorin Oakenshield and his company undertake their dangerous quest.
Smaug is not merely a beast guarding treasure. Tolkien presents him as intelligent, articulate, vain, suspicious, and skilled at manipulating others. Bilbo’s conversation with him in the mountain becomes a contest of riddles and psychological strategy, and Bilbo’s discovery of a vulnerable patch in Smaug’s armor helps lead to the dragon’s downfall.
The alternative names in this question belong to other Tolkien dragons or legendary objects. Scatha is a dragon of the Grey Mountains, while Glaurung and Ancalagon belong to earlier First Age stories. Smaug is the dragon specifically associated with Erebor in The Hobbit.