A monster in game is usually a hostile NPC/faction opposing the player.
Typically this is a animal-like creature of unusual size/shape/color.
Psychologically the more uglier monsters are perceived as emanating evil/corruption and are used for plot devices/story progression.
Visual style often dictates the purpose/background of monster.
Typical guidelines:
1.Monsters don't exhibit intelligence(except for boss monsters) and behave as member of species.
2.Size proportional to power. If it has visible weapon its size/type indicate the combat mode the monster engages in.
3.Color hints at element/power of monster.
4.Monsters can't obscure level features(unless part of plot) but can be hidden.
5.Monsters can't be spawned with player entrance/exit point.
6.Monsters can't interact with player beside combat and player-initiated actions(charm/taming/etc).