The creature Bilbo meets beneath the Misty Mountains in The Hobbit is Gollum.
Bilbo becomes separated from Thorin Oakenshield’s company and finds a strange golden ring in an underground passage. He then encounters Gollum, a solitary creature living beside an underground lake. The two play a riddle game, and Bilbo eventually escapes by using the Ring’s power to become invisible.
Gollum was once known as Sméagol, a Hobbit-like member of the Stoor people. The Ring has prolonged his life and distorted his body, mind, and desires. In The Lord of the Rings, his pursuit of the Ring continues and becomes essential to its eventual destruction. Gollum is sometimes confused with the giant spider Shelob or the dragon Smaug, but neither appears in Bilbo’s riddle-game scene beneath the mountains.