Monsters is the term that humans use for beings that belong to the supernatural plane of the world. These beings are a pivotal part to the Pareidolia world as their influences has shaped the characters' individual plots and motives and even effected them directly throughout the story.
The original name for Monsters is actually "Daemon", roughly translated into English from the ancient R
According to the Tome of Daemons, there are over 67 species of Monsters that roam this world on the supernatural plane. But of course, only a certain handful has been introduced.