What is the former name of the creature Gollum?

The creature Gollum was formerly known as Sméagol.

Sméagol was originally a Stoor Hobbit, a branch of the Hobbit-kind associated with the River-folk near the Gladden Fields. On his birthday, he went fishing with his relative Déagol. Déagol discovered the One Ring in the river, but Sméagol wanted it as a birthday present. When Déagol refused, Sméagol murdered him and took the Ring.

The Ring gradually corrupted Sméagol's body, mind, and relationships. His family eventually drove him away, and he hid for centuries in caves beneath the Misty Mountains. His later name, Gollum, came from the horrible swallowing sound he habitually made in his throat.

Déagol is therefore not Gollum's former name; he is the relative who found the Ring and was killed for it. Frodo and Bilbo are both Hobbits connected to the Ring, but neither was Gollum's original identity. Tolkien uses Sméagol and Gollum to emphasize the divided remnants of the character's former self.