Felix the Grouch

From Muppet Wiki

Felix the Grouch is an old friend who visits Oscar in Sesame Street episode 3891. Oscar is disappointed to learn that his friend Felix has become a neat freak, and he worries that Felix isn't a real Grouch anymore. However they argue so much that Oscar realizes that Felix is just as grouchy as ever.

The character of Felix is a spoof of the various incarnations of The Odd Couple, featuring two roommates -- the messy Oscar Madison (originated by Walter Matthau) and the neat Felix Unger (originated by Art Carney).