"In FNaF 4, after you beat Night 1, you're given the ablity to move around the house to explore. If you go to the room right from your bedroom, you'll find a room that I think is the brothers room, and you will see the Mangle lying on the ground, lifeless (or maybe not). Why is she/he there? My theory is that when the kid first visited Fredbear's Family Diner, he wandered away from the stage room and went into the backroom of the pizzaria, where he saw the Purple Guy stuffing dead children into suits (which is a theory on its own, explaning why he sees the animatronics in a nightmare state), but adding on to this theory, originaly thought out by MatPat from Game Theory (I think); while the child saw the Purple Man stuffing the dead children into suits, he also saw the prototypes of the Toy Animatronics, but the one that scared him the most was the Mangle, and just like how a child can easily get scared and be haunted by what he saw, the kid started having hallutionations about the mangle.
The short way of explaning it is that Mangle's a hallucination. |:("