Question How to retroactively apply prefab?

Say I have two prefabs, enemy and player. And I realized they have some parts in common such as Collider and Rigidbody. So, I make a new prefab called "Animatable". Then I want Enemy and Player to both extend from this Animatable. How do I tell Unity to make them both descend from this parent prefab without needing to manually re-make both of them?

 

I think the tool you are looking for is Prefab Variants. https://docs.unity3d.com/Manual/PrefabVariants.html

You can right click > Create Prefab Variant, or drag your premade prefab into the scene, and choose Variant instead. This will have a base Prefab that they will inherit from.

 

I have used prefab variants before but it seems to be different from the question I'm asking. How would I retroactively apply the prefab (or prefab variant) to the object which was made without using a prefab? In other words, we have an "unpack" option which dissociates an object from its prefab.. what about the reverse, where I made the object before I realized I'd want to make it a prefab, and want to newly associate it with a prefab

 

For the most part you can't. In some cases you can manually edit the prefab .meta files but it's messy, not very reliable and likely to break stuff. I have seen a couple of posts discussing that approach.

Starting with Unity 2022 they added some options for replacing the prefab of existing instance or reconnecting unpacked prefab. https://blog.unity.com/engine-platform/prefabs-whats-new-2022-2 It's better than having nothing, but it has it's limitations and you still can't change the base of existing prefab or a variant. Make sure you are using version control or have a backup of your project when trying out this feature.