I has a theory about Springtrap's first appearance.
The Stage01 minigame shows Golden Freddy and Springtrap on stage together. Could they have been the animatronics from Fredbear's Family Diner? Phone Guy calls these suits the "classic" suits in FNAF 3, and those messages were recorded before FNAF 1.
Assuming Golden Freddy has a hybrid suit, it makes it easier to picture the Purple Man in there, luring unsuspecting children into the Safe Room...
Then, when the ?ghost children spirits? confront the Purple Man, he hides in the Springtrap suit and becomes a mangle.
Not sure if the yellow colouration is patina of age or whether it was the original mascot's colour, but I do believe that Springtrap was the first Bonnie ever. These old guys were reimagined to create the withered animatronics from FNAF2, and then reimagined again for FNAF1.
If this isn't the first time someone's put this theory up, I'm sorry. But feel free to share your theories on the new old guy!!