This is a complete grasp at straws, but i had the thought, and would like to share it.

Nightmare, our undesirable depiction of our debonair Fredbear, is not just our final act, he's the whole show!

Nightmare Bonnie

Nightmare Chica

Nightmare Foxy

Nightmare Freddy

Nightmare Fredbear

Nightmare Springtrap

Nightmare

We know the animatronics get reused for parts right? You get decommissioned, it's the scrap pile for you! The withereds were suffering from this fate apparently, and Nightmare looks very withered.

Nightmare is really mostly just an endo-skeleton in disrepair. While he functions, he seems to be missing pieces and parts, \But what if his parts are being used in everything?

A piece of nightmare taken here and there, an eye here, metal rods there, a spring or two. This company doesn't seem to throw anything away, even going so far as to board up a suit behind a wall, rather than just throw it away.

So then, is it possible, that maybe nightmare was a haunted endo-skeleton, spreading his influence as his parts get used in other animatronics since the days of, or even before Fredbear?

Let me know what you think!