Denny's is a real restaurant chain featured in South Park. Its first appearance was in the Season Fifteen episode, "Crack Baby Athletic Association".
It was mentioned in "The Wacky Molestation Adventure" that Mark Cotner was the manager of Denny's in Breckenridge.
Denny's was seen in "Raising the Bar", when Eric Cartman entered the restaurant using his mobility scooter and received preferential treatment on account of his handicap.
Denny's appears again in "The Problem with a Poo", where Mr. Hankey speaks with Kyle to come up with a plan to get his job back after being fired as the director of the Christmas Pageant.
In "Nobody Got Cereal?", Denny's appears on the Channel 4 News as one of several locations where a ManBearPig attack took place.