Retargeting is to stop attacking something and attack another target. Some creatures can change their target, some do it only when the player they are attacking has moved out of range or disappeared (most weaker creatures). Most creatures that use distance attacks, as well as other creatures, will retarget to the closest player (most middle-strength and stronger creatures) and some creatures like Dragons, Cyclopses, Serpent Spawns and Giant Spiders seem to randomly change their target. This is dangerous for Mages or low level players that are hunting with a blocker.