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From the spell Animate Undead,
If you issue no commands, the creature only defends itself against hostile creatures.
A zombie is as smart as a mastiff while a skeleton is as smart as an ape.
What are some actions the animated undead might take in order to defend itself? Does it ready an attack? Does it move towards the hostiles to engage? I suggest these actions because animals of similar intelligence might do the same.
Animate Undead only creates two types of creatures, zombies and skeletons, neither of which are very smart (INT of 3 and 6 respectively), leaving them with few resources to figure out if a creature is hostile without that creature taking an obviously hostile action.
I don't know that comparing them to beasts of equivalent intelligence is really relevant either. The two creature types are fundamentally different, and would have different reactions to many different scenarios. Even different types of beasts that could be expected to be nearly identical could have wildly different reactions to creatures approaching them. Some beasts (like say, a wild wolf) could be incredibly territorial, and assume that anything that dares to enter their territory is hostile, while others (a domesticated dog) may assume that any creatures they come across are friends, and attempt to play with them, rather than fight.
Unless a creature does something immediately harmful, such as attacking them with a weapon or spell, the undead aren't likely to realize that the creature is hostile or even unfriendly.
So in this case, a "hostile creature" is one that directly attempts to harm the undead, or takes some other obviously hostile action.
How do these undead "defend themselves" against hostile actions? Well, typically with fighting back. To defend oneself is not to simply attempt to avoid harm, but also to attempt to prevent anyone who is attempting to inflict harm from continuing to do so. Think about this: self-defense laws are laws in place that can exclude you from punishment under the law for attacking another person, as long as you were attacking that person to defend yourself from their attacks. Zombies generally aren't known for their self-preservation instincts, so any attempt to defend itself would likely entail fighting the hostile creature until either it dies, the enemy dies, or the enemy runs away.
If one of the undead are attacked, they will fight back.
What mechanically happens when a Zombie "defends itself against hostile creatures." is up to the GM because these are not game-defined terms. This stance is further supported by the fact that the spell states:
[...] the GM has the creature's game statistics [...]
What the Zombie considers hostile, how it determines whether something is hostile, and what actions it does and does not take in order to defend itself are not defined in the rules and thus are all left to the GM
I am going with the opposite of what RevenantBacon suggests in their answer, but for the same logic.
A hostile creature is one that gets too close, looks at them 'funny' or points something dangerous looking at them
Undead, specifically zombies have really low intelligence, and that makes it difficult for them to work out what hostile really means. If you have ever tried to catch a wild rat you will probably find it fighting against you like it's life depended on it, even if your intention was to heal it's broken leg or something similarly benevolent.
They certainly won't wait for you to actively harm them.
As an undead creature, even with low intelligence they probably realise that everyone hates undead, and that makes them more likely to attack anyone who so much as looks in their direction and doesn't run away screaming.
They will certainly attack anyone who points a weapon at them.
How they defend themselves is by using whatever actions they have in their stat block
An animal would try and run away, and that is what I would suggest here if it didn't specifically say 'defends itself' because running and hiding isn't defending. Defending in most of the definitions I can find involves actively doing something.
The only real actions that they have in their stat blocks are the basic attack actions, so as soon as they feel threatened they would attack.
