A Monster is an unusual creature that differs from normal beasts due to appearance, habit, magical ability, or even intelligence. They have levels just like pretty much anything else capable of attacking, and can gain levels by killing each other or anything else.
Monsters can be found anywhere. There is no area completely free of monsters, though like beasts they tend to gravitate towards habitats that best suit their biology.
Boss monsters are much stronger than normal monsters of an equivalent level. A level 30 boss monster, for example, can easily defeat a level 30 normal monster.
For a more visible comparison, a normal level 10 monster can have around 1000 HP. A level 10 boss monster will have anywhere from 3000 - 7000 HP.