To Sweeney Todd, in the form of Muriel's "crazy barber" nephew Fred. The horror is negated somewhat by his cheerful rhyming and the fact that in the end, he's mostly harmless.
Also, stock footage of King Ghidorah, a recurring nemesis of Godzilla, appears several times throughout the series.
The ending to the original short features Courage holding open the iris out to comment "This shouldn't happen to a dog!". The iris then closes on Courage's nose, causing him to yelp in pain. This gag is a reference to the classic Bugs Bunny short "Hare Ribbin'". Which is further played with in the sequels. "This still shouldn't happen to a dog!"
Also, there are figurines of Dilworth's Dirty Birdy short in the house.
The twin raccoons from "Campsite of Terror" have the same design as Avery, a character from Ace & Avery, a segment on Big Bag which Dilworth worked on prior.
Nedd from Noodles & Nedd (a series of Sesame Street segments by Dilworth) often appears, most commonly as a framed picture in the Bagge house lounge room.
The word "Dil" often appears, this is referring to Dilworth.
The episode "The Mask" contains a few overall elements and themes of gritty exploitation and noir, especially the violently abusive gangleader boyfriend.
Mad Dog's defeat is rather similar to that of Sykes from Oliver & Company, being too caught up in his fight with The Hero to realize his car is heading right into an oncoming train until its too late, the crash itself happening offscreen.