Certain art files may contain animation for use with characters, creatures, or other objects. Within a creature or character you can define the animation it uses in different circumstances.
The game uses animation groups. The game plays these groups when the correct situation arises.
Creatures must all have their animation defined uniquely. That is, all creatures animate differently and use different art than other creatures. Characters, on the other hand, use a base set of animations that they all share. Characters can be given unique animations, which override the base animation groups.