In the game's first level, one of the bosses that can be encountered is a giant dead lizard that does not need to be fought, as indicated by its sole message upon encountering it:


While most of the monsters in the game are passing references to pre-existing monsters commonly found in popular culture, this monster is unique in that the same character design originally debuted in a 1988 Japanese toy model kit line called "Gegebo Majyu" (translated to English as "Pukey Evil Creature"), where it went under the name "Torigaran" (translated to English as "Chicken Carcass"). Gegebo Majyu was manufactured and sold by Bandai, who also published and released Monster Party exclusively in North America one year later in 1989.
Contributed by MehDeletingLater
The game's prototype, entitled "Parody World: Monster Party", vastly differs from the final game graphically. Most notably, the title screen featured blood instead of green slime, and many of the enemies and bosses were originally parodies of copyrighted movie characters or mythological creatures:
• The man-eating plant was originally Audrey II from the 1982 musical "Little Shop of Horrors" and shot quarter notes instead of bubbles.
• The Jack-O-Lantern was originally Caesar from the "Planet of the Apes" film series, and the background of the fight featured the famed image of the destroyed Statue of Liberty from the original 1968 film.
• The gorgon snake was originally a humanoid, most likely Medusa.
• The ghost man was originally a mummy.
• The giant spider was originally the titular antagonist of the 1982 film "The Thing".
• The giant cat-in-a-box was originally a gremlin from the 1984 film "Gremlins".
• The grim reaper was originally an alien robot resembling a Xenomorph from the "Alien" franchise.
• The vampire ghoul bore more of a resemblance to a stereotypical vampire.
• The projectiles released by the eggs in Round 2 were originally Facehuggers from the "Alien" franchise.
• The baseball catcher enemy was originally Jason Voorhees from the "Friday the 13th" franchise.
• The ghosts were originally "jiangshi": Chinese hopping vampires.

These graphical aspects were mainly changed both due to Nintendo of America's stringent censorship policy and the risk of copyright lawsuits, due to the United States' more strict copyright laws compared to Japan.
Contributed by game4brains
In round 7, when you defeat two of the bosses, you are rewarded with the key to the exit early. If you defeat the last boss of the round, the key disappears, leaving you stuck in the level. It is unclear if this is a glitch in the programing or an intentional design for the level.
Contributed by gamemaster1991