Hmm. I like that theory, Freddy being the one who bit the kid and Foxy trying to defend against the others.
My own theory is that Golden Freddy was the one responsible for the bite of 87. He only appears at a certain time and is a hallucination. His body is limp, as if the robotics inside have been taken out. If Foxy bit a kid, or Freddy, surely they would've taken out the robotics, to stop anything like that happening? This is why Golden Freddy is limp. He has no...skeleton inside him, unlike the others. They were scared of him and locked him away, and he's unable to move as a suit cannot move on its own.
To Mike. Golden Freddy is simply a hallucination. He's not really there. The message "It's me." Appears when he does come up. People say you were the kid who was attacked and you came back after several years to face your fears (which is why you die of fright when Foxy comes in as he's the scariest one) and when you see the poster of Freddy your mind tricks you into thinking its golden Freddy. You put the camera down in a state of panic and of course, your mind tricks you into seeing the one who attacked you. Golden Freddy. The message "it's me" appears because you know it was Golden Freddy who attacked you. You're scared of that, and so your mind shuts down (hence why the game crashes).
I do feel that the others are sometimes trying to protect you, and Foxy doesn't even touch you when he enters the room (I've always seem Foxy as male, myself. The new foxy however, is most certainly female) and you die from fright. Freddy gets in and you panic, trying to move. He panics, and tries to hold you down. Again, you die from fright and you're the reason there's bloody handprints on his face. Of course, the animatronics stuff you in a suit to try to "make you alive" but say you were still alive, you'd be shredded by the metal.
I always did think Foxy was the biter, or Freddy, but something about Golden Freddy just doesn't add up. Whys he limp? Whys he a hallucination? It's just my attempt at trying to think of his purpose really
