Very early in my Animation i decided that the Undead were not Zombies. However Zombies to exist just in this World the Undead and Zombies are different things.
For example the Undead hoards used by The Greater Knight are obviously undead corpses possessed by dark magic, but unlike Zombies these Undead are intelligent and able to do more than just eat Brains and groan. The Undead are able to stand straight and take orders from their superior who is normally the person who brought them to life. They are also able to talk and make structured conversations and judgements.
Zombies however in this World are the usual wandering eating machine. the Empire has set strict laws against creating Zombies since a Physician created a Zombie from a great warrior which in turn created havoc in the Realm. Unlike the Undead these Zombies show no sign of intelligence other than their need for food. They will attack any living thing including the one who created them.
As a conclusion, Undead are intelligant and Zombies are not. According to old tales there was once a village of Undead, however at some point in history this was mistake for Zombies and they were all butchered.