The '66 Bite

The '66 Bite was an event that happened towards 1966 in Fredbear's Family Diner, that was caused by a Golden Animatronic that accidentaly lured a child inside his mouth and resulted in a bite that caused the loss of the child's brain frontal lobule.

Theories
Fans have speculated and several Theories were made to unravel the Animatronic responsible for the bite.

The Animatronic Who Bitten the Child
In Five Nights at Freddy's 4 it was revealed through Mini-Games that the Animatronic responsible for the bite was actually Golden Freddy (in time, known as Fredbear in time), who was performing at The Stage when Sammy was accidentaly droped by his brother in Fredbear's jaw, of wich violently smashed Sammy's crane into pieces. After that, Golden Freddy and Spring Bonnie were both swapped to Parts and Service and later on replaced by The Four Original Animatronics.