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They could be tagged as "cowardly", "defensive", "territorial", or "belligerent". The cowardly ones would run away, while the defensive ones would fight back if they got attacked. Territorial creatures would attack if you got too close, and belligerent ones would attack on sight. Additional tags like "stationary" and "wandering" would help, as well as "predator", "prey", and "monster". These would work in combination, of course, so an ogre could be a wandering belligerent monster (roams around the map looking for somebody to kill) while a buffalo might be a stationary territorial prey (stays in one spot, but attacks if you get too close).