I've seen monsters fight each other.
As for the Mirrorknights, I've not seen that specifically. However I have noticed that all large upright standing bird-type mobs(bagoly, mirrorknight, urstixx?) tend to seemingly aggro from LONG distances, chase for long times, then sometimes just walk away for no reason. They act very strange and are honestly unpredictable. I've seen one come like I attacked it from across half the area to me, then stop, and kind of walk around before attacking.