In 1969, Jim Henson created a test pilot for the medieval-themed comic strip The Wizard of Id. Check out a clip of this Muppet show that never was.
Henson was a fan of the comic strip and actively sought to collaborate with the cartoon's creator. Says The Henson Blog:
In the summer of 1968, Jim Henson met Johnny Hart, the creator of the popular comic strips B.C. and The Wizard of Id. They discussed the idea of creating a television show based on The Wizard of Id that would combine puppets with an animated background. That fall, puppets were built, and Jim and his colleagues made a presentation to Hart's publisher, the Publishers-Hall Syndicate. The response was positive and in early 1969, Jim shot a test pilot. Robert Reed of Publishers-Hall spent the next year and a half trying to sell the show to the networks as a series or as several specials. In September of 1970, ABC expressed interest in making a Wizard of Id feature film, but, by that time, Jim was busy with Sesame Street, the Tales of Muppetland specials, and his variety show appearances and decided not to take the idea any further.
[Via Cartoon Brew]