often a type of grotesque creature

A monster, otherwise known as simply a mon, is a type of creature in fiction. They are usually found in legends or horror stories. They are often ugly and may make people and other creatures scared. They also eat people sometimes. A monster could be anything really, like a vampire or a witch or even a werewolf. Figuratively, a monster is a very evil person. Sometimes these creatures are not bad or monsters as you would say because some protect other people and/or are inoffensive.

Artist representation of a yeti or Bigfoot.