What was Gollum's name before he was possessed by the ring?

Gollum's name before he was possessed by the Ring was Sméagol, commonly written Smeagol without the accent. He was originally a Stoor, a branch of Hobbit-kind, living near the Gladden Fields in Middle-earth.

On his birthday, Sméagol went fishing with his relative Déagol, who found the One Ring in the river. Sméagol immediately wanted it as a birthday present; when Déagol refused, Sméagol murdered him and took the Ring. The Ring gradually corrupted his mind and body, eventually driving him into exile.

The name “Gollum” came later, associated with the horrible swallowing or gurgling sound he made in his throat. Tolkien also gave Sméagol a related Middle-earth name, Trahald, while “Sméagol” was designed as a translation connected with Old English ideas of burrowing or secretiveness.

Déagol is a common mix-up because he was the person who found the Ring immediately before Sméagol stole it. Déagol was not Gollum's original name; he was his relative and victim.