The Great Gazoo

The Great Gazoo is an alien exiled to Earth who helps Fred and Barney, often against their will. He is actually from the future, and is quite dismayed when he realizes he has been sent back to "the Stone Age". He can only be seen by Fred, Barney, Pebbles, Bamm-Bamm and other small children, Dino and Hoppy.

Trivia

 * He was played by Alan Cumming in his live-action debut, The Flintstones in Viva Rock Vegas.