Also a minor character in an episode of Animaniacs. Basically he was a short clown who would run onto the screen to the sound of a happy fanfare, then run off again. He did this several times throughout the show until the punch line, which went like this:
Enter clown
Exit clown
Dot: Who is that?
Wacko: Running joke
Sound of a discord indicating a deliberately bad joke.
It now occurs to me that he may in fact have been named Running Gag, not that it matters
rootbeer277 says re Running joke: The first season Darkwing Duck episode "Comic Book Capers" featured a pie-throwing indian named Little Running Gag.