In general, the monsters of Monstropolis are often diverse in appearance. Most monsters are often brightly colored and come in all shapes and sizes. A monster's head may be separated from its body by a neck, or they may be joined at the torso. Monsters typically have either two, one, or many eyes that are either separated by stalks or fixed inside their skull, a large mouth, and occasionally, a nose and/or one to many horns on their heads. Some monsters have hair on their bodies. The number of limbs a monster typically has varies, as with the limbs themselves: some have none, others have either two upper or lower, both, or many. A monster's limbs may either be hands, feet, claws, and/or tentacles. Some monsters appear to have tails. It is unknown if any monsters have skeletons, and even if they do, it's unknown where their bones end and their muscles begin.
Most monsters normally do not wear clothing, and even if they do, they would often be restricted to either hats, ties, glasses, and/or shirts but no pants.
Most monster given names, nicknames, and surnames vary from either both normal given and surnames (James P. Sullivan, George Sanderson, Don Carlton, Claire Wheeler, etc.), a normal given name and an abnormal surname (Randall Boggs, Henry J. Waternoose, Abigail Hardscrabble, Scott Squibbles, etc.), an abnormal given name and a normal surname (Lanky Schmidt, Claws Ward, etc.) or both abnormal given and surnames ("Phlegm" Bile, Fungus, etc.).
There are seven main types of monster races:
- Fungus (tall, thin monsters with long limbs like Lanky Schmidt, Jeff Fungus and Carla "Killer Claws" Benitez)
- Charlie (cephalopod-like monsters like Charlie Proctor, Noodles Rivera and Carla Delgado)
- Pill (monsters with pill-shaped bodies like Dirk Pratt, Brynn Larson and "Picasso-face")
- Slug (slug-like monsters like Roz, Smitty and the Slug Monster)
- Block (giant rectangular monsters like Joe Ranft, Earl "The Terror" Thompson and Percy Boleslaw)
- Spif (humanoid, nose-horned monsters like Chip Goff, the fire-breathing nosehorn and Nose-horn "George Sanderson")
- One-offs (no defined body type like Waxford, Donna Soohoo and Ricky Plesuski)
According to the DVD bonus video, History of the Monster World, all monsters evolved from creatures known as "Mons" that coexisted with humans, until one day the humans developed weapons and drove all of the mons from the mainland and forcing them all to live on an unknown island (where Monstropolis is implied to be located) where they ate strange fruit causing them to grow bigger and scarier and becoming the monsters we know today.