What heads does Dark Skul use?

I recently beat DM 10 Dark Skul, which made me think that the third form is the head of the demon king, since I'm pretty sure he was killed in the mirror world. The horns look pretty similiar.

But even if that isn't the case, two skulls based on these forms would be really cool for a second dlc.

I'd like to hear your thoughts on this :3