I think it's a hallucination too because the other animatronic would've stuff it into a costume by then.
but it makes me wonder if it is real. could it be a prototype of foxy cause the ears are at a similar angle and shape as this endoskeleton. they just enlarge his ears on the final product. or maybe a new addition for the show stage
basically either Scott wanted us to know what the new endoskeleton in the new animatronics look like or just redo its look cus the old one were plain looking so he put it in there but just as an easter egg even though it may scare us when we see it.
though some theorized that it's the endoskeleton from golden Freddy as his original name was spring Freddy from Freddy's Family diner. you can see a bare endoskeleton in the first game too. in the parts/service room on the table if you were to brighten the image. they say that because when you see golden Freddy he's not standing he's sitting on the ground all limp not moving at all. And if you look closely you don't see the inner set of teeth. He's just an empty costume. the diner was where spring Bonnie later known as springtrap was originally from as seen in one of the hidden minigames where you see spring Freddy (aka golden Freddy) and spring bonnie (springtrap)