During the scene where Flim-Flam and Scrappy pretend to be detectives arresting the gang, Scrappy introduces himself as "Elliot Messy," a nod to Elliot Ness, though the character was a Treasury agent, not a detective.
Before leaving the room, Flim-Flam does a disclaimer:
When Daphne, Shaggy and Scooby are sucked into the demon mirror, they are shown in a room with several staircases going in multiple directions including upside down, it alludes to M.C. Escher’s well-known piece of art titled "Relativity."
"The Ghouliest Show on Earth"
Episode title is based on The Greatest Show on Earth. The episode title also alludes to the well-known phrase "The Greatest Show on Earth", which was the slogan of the Ringling Bros. and Barnum & Bailey Circus that started in 1870.
Professor Phantasm bears more than a few similarities to sinister master G. W. Dark of the Cooger and Dark Circus.
At one point, the President of the United States himself urges Scooby to rethink his retirement rand ejoin the team. Not only is the president designed after Ronald Reagan, but on his desk is a jar of jellybeans. Reagan was famous for loving Jelly Belly brand Jellybeans.
"Scooby in Kwackyland"
"Kwackyland" is based on the whimsical world of living objects, Wackyland, consisting largely of surreal, Dali-like imagery and free-floating objects that obeyed no laws of standard physics in Porky Pig episodes "Porky in Wacky Land" and "Dough for the Do-Do", as well as being a major part in Tiny Toon Adventures, where Gogo Dodo is from there.
"Kwackyland" could also be a parody of Disneyland.
Other than Vincent Van Ghoul and Flim-Flam, in "That’s Monstertainment" Shaggy, Scooby, Daphne and Scrappy are given recurring roles in the Frankenstein movie mythos: Scooby Doo as mad scientist Dr. Frankenstein, or in this case, Dr. Frankenscoob; Scrappy is "Egad", which is a parody of "Igor"; the misshapen servant and lab assistant to Dr. Frankenstein; Daphne is both Dr. Frankenstein’s fiancée, Elizabeth, with her dialogue’s indication and the Bride of Frankenstein with her hair and appearance; and Shaggy’s role of Constable Von Shaggobon is a reference to the recurring role of the concerned lawman who goes to investigate what the doctor is up to when the townspeople start to get worried about his unnatural work.