The Muppet Show 1x19

Muppy, the favorite pet of The Muppet Theatre owner J. P. Grosse, becomes a prima donna when Kermit reluctantly allows him to do a musical number with Scooter. Muppy even gets his own dressing room and star billing, but Kermit draws the line when the dog asks to change the show's title to The Muppy Show. When Muppy locks himself in his dressing room and refuses to perform, Scooter does the number with Fozzie instead.