This came to me after many hours of my day being spent listening to a fnaf song playlist i made on youtube and one particular song got stuck in my head but i have a strongly based theory on who caused the bite of '87.
Who was the single most abused animatronic that existed at the time the event occured, one poor animatronic that was left to be dismantled and reassembled by children as they saw fit, the staff had all but given up on trying to fix poor mangle and one would imagine that a semi sentient being such as the animatronics would eventually get sick of such things.
My continued support for this theory lies in the fact that despite FNAF 1 being based after the bite of '87 none of the toy animatronics seem to exist this being because they were scrapped after one of them snapped and bit a patron, of course after this they couldn't keep that line of animatronics incase their program was also faulty.
Unlike the four animatronics inhabited by the childrens spirits the mangle did not have such a thing to get in the way of it harming a child (assuming the bite was on a child with this one) it is my belief that the inhabited animatronics would not harm a child as they themselves are in a way children.
Scott has not made any reference to the fact that it could have been one of the toy animatronics and i feel that this in itself could be an indicator that my theory is right on the money why would he give us an actual lead and risk ruining a potentially game making plot twist.
ok i have had my say here and i would like to hear from you guys what do you think, either try to discredit my theory or just comment on it... even better if you have more supporting evidence share it with the world show scott we can't be mislead and out smarted.