A character from the original game. The Mouth Monster is a large, brown creature that makes dog-like sounds as it surveys the Barracks Settlement. Should Madotsuki approach it, it chases after and consumes her if it comes close enough to do so.

Using the Plank with the wooden cage in the center of the settlement allows it to be propped open. If the player sets the Red Eye in the cage, and the Mouth Monster comes close enough, it will enter the cage to consume the eye. Should Madotsuki close the cage, the Monster becomes trapped, allowing the Pirori to roam the Settlement freely.