I don’t understand what’s unclear. Maybe the confusion is about what an “actor” is, as opposed to a “mesh component” or a “mesh asset?”
An “actor” is a game object that has a certain behavior, can get replicated to networked clients, and so on.
An “actor” doesn’t, by itself, show up on the screen – it needs to use some kind of mesh.
If you want an “actor” to look different, you can change the mesh it uses.
Meshes are typically rendered by a “Static Mesh Component” or a “Skeletal Mesh Component,” which reference a mesh asset of the right kind (Static Mesh or Skeletal Mesh.)
Thus, to make the “enemy” actor render “a squished goomba” instead of “a full-size goomba,” one of the ways of doing that is to replace the Skeletal Mesh in the Skeletal Mesh component from a “full size” mesh asset, to a “squished” mesh asset.
If the squished mesh asset has a different skeleton, you’ll also want to change the animation blueprint to match.
(If you use static meshes instead, the static mesh components has not direct animation, so that doesn’t matter there.)
If you have an AIController, the AIController should probably know whether it is “squished” or not, to stop moving, for example. Or if your enemy subclasses Character, you can set the movement mode to custom, and max speed to 0, which will make it not move.