The creature originally called Sméagol in J.R.R. Tolkien’s legendarium is Gollum.
Sméagol was a river-folk hobbit-like being who discovered the One Ring while fishing with his relative Déagol. Sméagol killed Déagol to obtain the Ring, and the Ring’s corrupting influence eventually drove him from his community. He lived for centuries in the Misty Mountains, where his speech and repeated choking sound led to the name Gollum.
Gollum appears in both The Hobbit and The Lord of the Rings. In The Hobbit, he is the mysterious cave-dweller who challenges Bilbo to a riddle game. In The Lord of the Rings, his divided relationship with the Ring becomes central to the journey toward Mount Doom.
The names refer to the same character at different points in his history. Sméagol is his earlier personal name, while Gollum is the name associated with his transformed, isolated existence and the sound he habitually makes.