Gollum’s original name before he possessed the One Ring was Sméagol. He was a member of a river-dwelling people related to the Stoors, a hobbit kindred, and lived near the Gladden Fields.
On his birthday, Sméagol went fishing with his cousin Déagol. Déagol discovered the Ring in the river, but Sméagol wanted it and killed him to take it. The Ring’s influence and Sméagol’s own corruption eventually drove him away from his people. He later lived beneath the Misty Mountains, where he became known as Gollum.
The name Gollum reflects the strange swallowing sound he repeatedly made. Tolkien presents Sméagol and Gollum as connected aspects of one damaged character rather than as entirely separate people. Déagol is sometimes mistaken for Gollum’s earlier name, but Déagol was the cousin who found the Ring first.