There werewolves in the Karanas have this odd aggro where they will attack mercenaries. The player character can be invisible and the werewolf bandit and werewolf attack everything except merchants. The exceptions to this may be the ogres, the wandering spirit and Oobnopterbevny Biddlets. I know I have also seen the aforementioned gnome killing the cyclopes in the zone.
I know there are other examples but this seems worth mentioning as it is unmentioned so far for this particular mob type in the old world of classic eq.