Entities living in the human world. They are most likely ancient animals or nature spirits with supernatural abilities. However, some are mutations of animals or the result of a gene experiments. Monsters are classified based on their species and attributes.  Examples of Monsters are dragons, trolls, yeti and goblins. Unlike demons, exorcism doesn't leave any effect on them except for monsters with demon genes inside them. Without the presence of demons after the Great War, the monsters get much stronger and monster population increased rapidly. Some even evolved to a dangerous level that threatens other living beings and the ecosystem. Besides that, some monsters possess high quality materials and can be sold at high price in the market or other logical purposes.