Springtrap AKA Spring Bonnie is former child-murderer turned undead monster trapped inside an animatronic and is the main antagonist of Five Nights at Freddy's 3. He is also the only animatronic who can kill the security guard unlike the Phantom animatronics.
Springtrap is a brownish-golden color resembling Golden Freddy. Its "fur" or synthetic skin is rotted, and it has copious amounts of external damage. It bears human-looking eyes, and a deranged smile, likely due to decades of neglect and general wear-and-tear. Behind its mask lies a hidious mummified face with a look of frozen dread.. Dried and crusted prices of brain and intestine are visible. The synthetic skin has also rotted off the feet entirely, exposing metal feet with veins across them. It resembles both Bonnie and Golden Freddy in many aspects. This mysterious animatronic also has a very spastic, convoluted way of moving, and a loud, electric-sounding scream, like the other animatronics. It also walks in a very fluent, casual way, more like an organic being than a lifeless robot.
Springtrap was a prototype animatronic suit, designed both for endoskeletons and humans. However, the suit's spring-locks were loose, and malfunctioned easily, causing the suit to contract and tighten if ever they did, crushing any human inside, thus deemed extremely dangerous. For this reason, Spring Bonnie was abandoned in place of the current animatronic suits and left to rust away.
In the games
Five Nights at Freddy's 3
Many years later, the creators of Fazbear's Fright: The Horror Attraction found Springtrap's suit and decided to use it as as a part of the attraction, unaware of the suit's sentience.
At the end of the 5th night, it is revealed that Springtrap was the suit that The Killer attempted to get into upon being cornered by the spirits of Freddy and the others. While he did succeed at hiding in the suit, rain caused the springlocks to loosen, and the endoskeleton to force itself into the killer, its inner workings eviscerating The Killer's body and killing him. The Killer died within Springtrap and his corpse remained inside (as shown in multiple images at the game's start up menu). The Killer's zombified corpse remains trapped within Springtrap in the years prior to and including the advent of Fazbear's Fright, and his conscious likely haunts the suit in a similar fashion to the souls of the murdered children.
At the end of the 6th and final night, the entire building burns to the ground from unknown causes, and Springtrap himself is presumably destroyed for good along with it.
However, in the newspaper article, it is mentioned that the remaining parts of anything found within the attraction would later be auctioned off. When the image is brightened, Springtrap's head can be seen in the background, although it is unknown whether it is just the head, or if more of the frightful animatronic remained.
Five Nights at Freddy's 4
Springtrap appeared again in the minigames of Five Nights at Freddy's 4. It was revealed in one of the mini-games that Springtrap, who was rather called "Spring Bonnie" was present during The Bite of 87 which was possibly caused byFredBear after Big Brother and the bullies grabbed the boy to get him close into Fredbear's mouth for a "kiss" but the prank went wrong and the boy's head got crushed when FredBear suddenly closed his mouth, which resulted in the boy losing his frontal lobe.