One of the most popular theories is that he the only animatronic that is not a phantom - Bonnie.
I do not believe Springtrap is from the other games.
Let's take a close look at Springtrap ...
Please take note of the eyes, the nose, and the button.
Bonnie from 5 Nights At Freddy's 1 did not have buttons, nor did he have eyebrows. His nose was also larger than Springtrap's .
Bonnie #1 from 5 Nights At Freddy's 2 *did* have buttons, however the animatronic was already torn apart ! Please note the shape of the "stomach" patch does not match Springtrap's .
Bonnie #2 ( Toy Bonnie ) from 5 Nights At Freddy's 2 has absolutely nothing that matches Springtrap
From what I have seen, I think Springtrap is a completely new character.
According to the 'new' phone guy (day 2), he was discovered in an extra room.
According to the 'Vintage Training Audio Tapes', the old 'spring suits' were sealed off in the 'safe room' due to 'legal' reasons (nightmare mode).
According to the mini game end of day 5, Purple Man was killed in a rabbit spring suit - it was also hinted at in the night 5 'Vintage Training Audio Tapes'.
From all this, I believe that Springtrap is an experimental animatronic that was sealed away in the 'safe room' [*1] after killing Purple Man.
[*1] It is unclear which 'safe room' this was at. According to the 'Vintage Training Audio Tapes', there were many Freddy Fazbear Pizza locations .