Gollum is a slimy, skinny creature that Bilbo meets is the tunnels of the Misty Mountain.

      Gollum was once a Hobbit just like Bilbo but he found the Ring of Power that was built in Modor (more about this in The Lord of the Rings). The ring possed Gollum and he changed. 

     When the dwarves and Bilbo, and Gandalf reach the Misty Mountain they are found by Goblins. 

     Bilbo falls behind and goes down a tunnel that goes to a lake under the mountain. There Bilbo meets Gollum and says riddles to try to have Gollum show him the way out.

      Gollum loses the ring and Bilbo takes it. Gollum gets mad and tries to kill Bilbo. Bilbo escapes and finds Gandalf, Thorin, and Company.