I never really gave this much thought until this question brought it up: Is a dead creature's body considered to be an object? And is it still a creature?
My initial reaction was that it would not be an object, just a dead creature, since objects and creatures are defined differently. However there doesn't appear to be any strict definition of what an object or a creature actually is. And yet both creature and object are two terms that are used numerous times throughout the rule books.
Are these two terms mutually exclusive or can something be described as both?
Constructs for instance are the construction and animation of an object, but are they considered to be both an object and a creature, or are they one or the other? Is the meat suit of a once living, flesh and bone creature still a creature after it dies, does it become an object, or is it both?
Furthermore, what is a corpse in relation to spell targets?
Spells tend to define targets in terms of choosing a creature or choosing objects. Revival spells, such as Raise Dead and Resurrection, specifically target dead Creatures, whereas spells such as Animate Object specifically target objects. To quote part of Animate Object's description:
Objects come to life at your command. Choose up to ten nonmagical objects within range that are not being worn or carried. […] Each target animates and becomes a creature under your control until the spell ends or until reduced to 0 hit points.
So if creature and object are not mutually exclusive and a dead creature is considered an object, could Animate Object be used on a corpse?
Best Answer
The answer is yes, Animate object would work on a corpse. The exact effect would depend on the size of the corpse.
While there are specific defined terms in D&D 5e there are also a equal number of that rely on what the word means in English.
Object - a material thing that can be seen and touched.
Creature - an animal or person.
However there is a caveat. In various effects, powers, and abilities. The D&D 5e rules are consistent in referring to creatures as things that are living or animate. Objects as inanimate things like tables, chairs, rocks, books, feathers, etc. It not spelled out but it is consistent.
The things to remember is that D&D 5e rules are not to function as a wargame. They do not define the boundaries of what is possible during a campaign. The setting is what defines that. Instead they are a tool to aid the referee in adjudicating the action. For example the description of humans don't spell out every detail that could come up. The mechanics about humans are those that the authors feel that are useful or come up often. The important of which is the effect being human on character creation. The author expect referee to use what they know about humans to adjudicate anything that the rules don't cover because it is implied that humans in a D&D setting are just like people in real life only living in that world.
One implication of this is that animate objects doesn't change any other physical property of the object other than to animate with the stats provided. If you were to say animate a block of salt, possible considering what salt miners carved out of their mines, and it was to walk into water, then it is reasonable to rule that it would be affected adversely as salt dissolves in water. Perhaps by treating water as a acid attack on the animated object.
So a corpse animated as a object would still be a corpse and subject to decay, smelling bad, etc. It would not gain the benefits of being undead although at first glance it would be hard pressed for a character to tell the difference. One area where I can see the difference being important is trying to animate a skeleton. It is reasonable to assume that the various create undead spells joins the bones together to form a complete animated skeleton. While a long dead skeleton is merely a pile of separate objects of bone.
For stuff that has no real world analogue, elves, magic, etc. The authors expect the referee to fall back on their knowledge of the fantasy genre. Because the implied assumption that D&D is being used to depict a fantasy setting. Which is why they included a list of inspirational works in Appendix E on page 312.
In fantasy it is tradition for some spells to work on anything, a lightning bolt doesn't care if its target is a person, animal, or a piece of furniture. Some spells to only work on people, for example charming or enchanting a princess. And other spells to work only on objects, like the animated furniture from Fantasia.