Question

Why did the cyclops believe his name was nobody?

Answer 1

The cyclops, Polyphemus believed that Odysseus's name was nobody because the cyclops could see no one, and Odysseus' said his name was Nobody. 

Answer 2

The cyclops in the odyssey he belives that odysseus was named nobody because he told the cyclops his name. This made it easier to kill the cyclops. So when other cyclops come he'll shout " Nobody is hurting me" or " Nobody is killing me.