You can share animations between different skeletal meshes (with different proportions and extra bones) without copying them (and making a rig) if the skeletal meshes use the same skeleton asset. This will let you reuse them at runtime.
Animation Retargeting | Unreal Engine Documentation
Using Retargeted Animations | Unreal Engine Documentation (under “Retargeting Using the Same Skeleton”)
Skeleton Assets | Unreal Engine Documentation
Though, from what little I’ve seen on Unity’s humanoid system, it’s not 1:1 (no muscle controls).