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Timmy Monster has been performed by Steve Whitmire, Dave Goelz, and Jim Henson.
Timmy is a full-body blue-green monster with green lips, green eyelids, a green nose, and gigantic feet. He first appeared in the background in Episode 117: Ben Vereen, during the opening and in the closing number, Pure Imagination.
In Episode 117, and in some publicity photos (and the cover of the first Muppet Show album), he had green hands, eyes without eyelids, and some kind of goggles or antennae over his eyes. Starting in the second season, his eye antennae were removed, he had eyelids, and he had green hair. Later in the second season, his hands were changed to be the same color as his fur, he no longer had hair, and he now had a tail.