1.21 Skeleton Wolf

Essentially, this could be a simple Easter egg type of mob, where after being stuck by lightning a regular wolf would become a skeleton wolf.

This could be a kind of nod to the old DanTDM mod videos for his Skeleton Dog Grimm.

To keep the game consistent, the wolf would have to be ignored by the wither and be healed with instant damage like the rest of the undead mobs.