Springtrap is a form that Dave or Old Sport will be transformed into if sealed in the saferoom. It happens in the Premature Ending, at which's end we get a nice caption of Dave as Springtrap with a middle finger raised up which is him pretty much saying "Fuck you, you finished the game too early".
Springtrap is a yellow/olive green heavily damaged version of Bonnie. He has human organs in his suit, and he looks like he's always grinning, because the material surrounding his teeth has deteriorated. A great portion of his right ear is also missing. Instead of dark pink in the original game, the human organs in the suit are purple in DSaF however if Old Sport is killed in it then the organs will be orange.