Monster is the general term used to describe anything non human. Although most monsters are hostile to humans, there are a few exceptions (the plantpyre Rosa and plant monster Scrufflebeck for instance).

As it stands the various species of monsters all hold each other in a delicate balance: zombies hold back mutants, mutants and plants hold back the machines, mutants rely on the plants for food, while were it not for the machines the plants (needing only sunlight to survive) would have conquered the planet, while zombies and vampires live off of humans.

Monster Types