Mangle

Mangle is a mangled animatronic in Kid's Cove at Five Nights at Freddy's 2.

Mangle caused The Bite of '87
This is a theory where almost everybody believe before Five Nights at Freddy's 4. Since Freddy and Foxy was in Parts & Service, Mangle the only possible animatronic, with his/her big jaw and his/her insane malfunction.

Mangle was haunted
Right now this has less to none evidence, the evidences is about the kid in Mangle's Quest haunted him/her, or he/she appear as a phantom in Five Nights at Freddy's 3.