Gollum’s original name before he found the One Ring was Sméagol.
Sméagol belonged to a river-dwelling people related to the Hobbits. On his birthday, he went fishing with his relative Déagol, who discovered the Ring in the River Anduin. Sméagol became obsessed with the object and killed Déagol to take it from him.
The Ring’s influence and Sméagol’s growing corruption eventually drove him away from his community. He hid beneath the Misty Mountains for centuries, where he became the creature known as Gollum. His repeated swallowing and “gollum” noises helped give him his later name.
Tolkien presents Sméagol and Gollum as connected identities rather than entirely separate beings. Sméagol’s remaining desire for friendship and repentance becomes important when Frodo and Sam encounter him. The character’s earlier name is sometimes confused with Déagol’s, but Déagol was the relative who found the Ring.