HOLY stuff WAIT. The check at the end of night five read 1987. The same year as the bite. Maybe FNAF 2 takes place before FNAF, and the one that bit a kid was the mangle. Just think about it. There is no mention of the killer of the five children in the sequel. It is mentioned though, that someone used a yellow freddy fazbear suit. This might be the suit used by the killer. In the newspapers in the first game, parents noticed blood and mucus on the animatronics. Maybe the animatronics that had the blood and mucus on them, the ones that the killer used to hide the bodies, were the toy animatronics. And after the bodies were found, the toy animatronics (and the mangle because of the bite) were disposed of by Fazbear's Pizzeria. To replace them, they repaired the older ones and put them on stage. This also explains why the phone guy is still alive. It makes sense because on night 6, the phone guy says he'll do the night shift next. Then after the phone guy got killed, (right before FNAF 1) They hired Mike. This theory also makes sense when you see that The Puppet was in front of freddy in that cutscene that took place in the FNAF 1 building.
