Monsters are hostile entities that roam the ship in Monstrum. Each time the game is loaded, one of the three monsters is randomly selected to spawn for that playthrough.
Monsters wander about the ship searching for the player and will aggressively begin pursuing them upon sighting them (marked with an audio cue unique to each monster).
A bipedal behemoth that appears to be infused with fire or lava. It has very loud footsteps, emits light from its face, and is notoriously fast during a chase.
A slender, almost skeletal entity with large, clawed fingers. It primarily travels through the ship's ventilation system and may cause egg sacs to appear throughout an area.
A demonic, humanoid creature surrounded by a purple aura. It possesses an array of telekinetic powers that enable it to paralyze the player and "lock" doors from afar.
- All three monsters may have once been human. The Brute's hands, feet, and left arm are very human-like, the Fiend appears to have human ears that have been "melted" or fused into its head, and the Hunter has a mammalian skeletal structure and human feet.
- Note #12 suggests that the Hisa Maru was being used to transport mysterious "specimens", unbeknownst to its crew. These specimens apparently pose a "risk from exposure" to those nearby, even when properly contained. Whatever those specimens were, it could be that each of the three monsters are the result of an infection, infestation, or mutation of one or more crew members of the Hisa Maru, or of the individuals involved in the operation of smuggling them aboard, due to said exposure.
- Only ONE monster may spawn per playthrough.
- Each monster represents a different fear; The Brute represents raw physical danger, the Hunter represents a fear of being stalked, or a sense of paranoia, and the Fiend represents a fear of the insane and unpredictable.
- Though nothing is planned currently, according to an interview done by IGN's Tom Bennet, Team Junkfish has stated that the possibility of a 4th monster is not entirely out of the question, as they currently have a "half-finished monster" in the form of Sparky, the original prototype monster that eventually became the Brute.