J.R.R. Tolkien gives Gollum the original name Sméagol in The Lord of the Rings.
Sméagol was a hobbit-like member of a river-dwelling people. He discovered the One Ring after Déagol found it in the River Anduin. Sméagol murdered Déagol to obtain the Ring, and the Ring’s corrupting influence eventually transformed him into the creature later known as Gollum.
The name Gollum comes from the choking, swallowing sounds he makes. His two names reflect the divided history of the character: Sméagol refers to his earlier identity, while Gollum is the name associated with his long life of isolation and obsession beneath the Misty Mountains.
Déagol is not Gollum’s original name. Déagol is the companion who finds the Ring and is killed by Sméagol. That relationship is one of the key facts explaining how Gollum first came to possess the Ring.