So a bit of backstory to the question. One of the players in my game is a death cleric and enjoys having a small army of zombies accompany him. We had come to a situation where it is not plausible for him to keep the zombies with him for the 24 hour period, but he spent the change (on a government writ) to get them locked up until his return, so he can try to regain control over his minions.
Now to the question. Is it possible to cast Animate Dead on a Zombie and have the spell work as normal. Is a zombie considered a corpse, technically? Also, if you say that it should work in theory, here comes the sticky part. He also wants to use this spell on a Mummy Lord (that he doesn't know is a Mummy Lord, yet). Does it simply not work since it's an undead creature and not a corpse?