OH F*CK

They were hallucinations created by CC or BV's mind after the reoccurring bullying from his brother and his friends. Since they wore masks, Nightmare Freddy, Bonnie, Chica and Foxy had been based on them. I can't say much about Nightmare Puppet, BB, Plushtrap and those guys, but Nightmare and Nightmare Fredbear either occurred after the bite or before, most likely after. They had formed in his mind during the coma, if this was after the bite. Sorry, I'm deep into the lore. 😭

In the fnaf 4 game, they are nightmares quite literally. BUT, my theory suggests that the nightmares are based on the original springlock animatronics

Nightmare Fredbear definitely is, since there was no real reason to have a fear of him until after the bite, since CC/BV died to him.

They are nightmares of a child

Oh.