Angus McGonagle

From Muppet Wiki

Angus McGonagle, the Argyle Gargoyle, gargles Gershwin gorgeously. He was originally booked as the guest star on episode 417 of The Muppet Show but was bumped for the cast of Star Wars when they showed up at the Muppet Theater. He eventually managed to get on stage, where he was joined by Mark Hamill in gargling "Summertime."

Angus also appeared in the background during "Singin' in the Rain" in episode 501 and as an audience member in Muppets Tonight.