Zetoxian

The Great Gazoo is a small green humanoid alien who was exiled to Earth in prehistoric times and made friends with the local "cavemen", or primitive Humans. He was banished from his homeworld of Zatox as a punishment for creating a doomsday device, so powerful that it could possibly destroy the universe. Gazoo claims that he never intended using it, although. Gazoo is only seen by his cavemen friends Fred and Barney. The other characters are apparently unable to see him because they don't believe in his existence. He also has the ability to float in mid-air.

Behind the scenes
The Great Gazoo was a regular character in the last season of the show The Flintstones. He also made cameo appearances in the Flintstones second live-action movie Viva Rock Vegas, and in the Warner Brothers show Duck Dodgers. A similar character, Ozmodiar, appeared in a couple of episodes of The Simpsons.