Springtrap is both the main antagonist and the only true animatronic in Five Nights at Freddy's 3.

It is the only enemy that can kill and end the player's game, while the phantom animatronics can only jumpscare the player and impair their vision and hearing.
Appearance
Springtrap is probably an early Bonnie suit (he is referred to as Spring Bonnie by the Phone Guy). It is a heavily damaged rabbit animatronic of a tarnished yellow-gold color. A great portion of its right ear is missing with wires sticking out of the stump left behind.
Springtrap's entire body is torn and tattered with gaping holes exposing caging and wires throughout, much more macabre-looking than any of the other animatronics from previous installments. While both hands and feet are attached and generally intact, its legs are entirely stripped from the mid-shin down, fully exposing endoskeleton metal and wiring. Both of Springtrap's eyes are properly set inside its head and it does not typically open its mouth - instead, Springtrap wears a permanent wide grin as the material surrounding its teeth has deteriorated. The exception to this is during its second jumpscare and in its rare boot images.
Close examination of Springtrap and the rare boot images depicting it show that there appears to be human remains within the suit. When viewing the torso and other exposed sections of the endoskeleton, there are chunks of dark pink material that resemble human innards. In the boot images, Springtrap has its jaw open and is pulling the suit head up, revealing a mummified-looking human head. The jaw and teeth of the head can be briefly seen during its second jumpscare.
Unlike the robotic movements of all of the previous animatronics, Springtrap moves in a fluid manner, slinking into The Office in a human-like fashion.
History
Most other animatronics have endoskeletons and support structures that makes it nearly impossible for them to be worn like a suit (without causing serious injury or death). Springtrap, on the other hand, was created to be a hybrid between an animatronic and a wearable mascot costume. According to Phone Guy's various calls throughout the week, the mechanical parts inside of Springtrap were designed to be tightly compressed against the outside of the suit, using spring locks to hold them in place, allowing someone to enter and wear the suit. However, anyone in the suit would need to be very cautious, because in the event that the spring locks malfunctioned, all the compressed animatronic parts would quickly shift back into place, severely injuring (and likely killing) whoever was inside the suit. Springtrap's spring mechanisms were faulty to the point that breathing on them lubricated them and made them malfunction, thus making it extremely dangerous to wear and leading to Springtrap being put away in an emergency/storage room in which all customers and most employees were not allowed to enter. This is most likely how Springtrap earned its name, trapping the user inside of itself because of faulty springs. The entrances to the rooms mentioned above were later covered up with pseudo walls and hidden from all camera views and maps, thus making these rooms nonexistent to unsuspecting customers and employees.
Purple Guy Death Gif 3
Springtrap's sprite when sitting down in the Night 5 Minigame.
When the Purple Guy returned to the restaurant to dismantle the animatronics, he awoke the ghosts of the five children he murdered. Terrified by the spirits, Purple Guy ran into the safe room where Springtrap was placed for storage. He wore the suit to hide from the ghosts, and the trick appeared to have worked at first, causing him to stand up suddenly and laugh at the ghosts while inside Springtrap. However, the mechanisms within Springtrap malfunctioned and Purple Guy was crushed and sliced by the internal springs and gears, thus killing him. Phone Guy's tapes also foreshadow this, as he explains that the person wearing the costumes must not breathe on the spring locks or make sudden movements, and Purple Guy broke these rules; he made a sudden movement by jumping into the suit, and possibly breathed on the spring locks when he laughed, resulting in the locks becoming loosened and his brutal death.
30 years later, Fazbear's Fright: The Horror Attraction was opened as a homage to The Missing Children Incident and other incidents that made Freddy Fazbear's Pizza infamous for being haunted. Employees found Springtrap and reused it for the attraction. However, at night, it roams the halls, presumably possessed by the body and spirit of the Purple Guy.
Locations
Springtrap enters every room and vent, and therefore can be seen by every camera unless it is in The Office, hallway, or outside The Office's doorway.
Behavior
Springtrap acts in a very human-like manner, teasing the player to its presence as if it were hunting them. It will go to more secluded areas of a camera area, seemingly attempting to hide from the player, or outright staring into the camera.
When around The Office, Springtrap will both slink around and even stare at the player through the window. On occasions when it is about to kill the player, it will hide right behind the door, but will peek in, simply staring the player dead in the eyes.
As mentioned in the Night 2 phone call, Springtrap is susceptible to audio distractions and will constantly follow the source of the noise the player creates, possibly due to it mistaking the noise for the sound of actual children. It is also susceptible to the vents being sealed, but will use them commonly as a means of getting to the player.
Springtrap may also be attracted to the sound of The Office's alarm blaring (when the light flashes red in The Office).
Audio
The sound Springtrap makes when attacking the player.
Play sound
Scream3
Trivia
He is one of the two animatronics to have their starting location out of sight and boot up in Night 2 the other being Foxy in the second game.
Occasionally, when the game starts up, one of three images may appear of Springtrap revealing the Purple Guy's mummified corpse inside of him, similarly to the eyeless Bonnie screen from the first game and the eyeless screens from the second game.
Upon analyzing the three images that can rarely appear when starting a night, it can be seen that Springtrap had no animatronic eyes, but instead has human eyes, though they could be Springtrap's eyes and were simply jammed into Purple Guy's skull when the spring locks failed. This can be supported by the fact that Springtrap's eyes glow, a trait not usually found in human eyes. It's possible that Springtrap's eyes are actually inside Purple Guy's, making his eyeballs glow. It's also possible Springtrap's eyes are glowing because of a paranormal reason.
Part of the killer's lower jaw can also briefly be seen in Springtrap's right-side jumpscare, and while he is in sight on CAM 15.
There is a chance to get double jumpscare right after the Phantom's jumpscare, Springtrap will jumpscare the player.
Springtrap hiding spot is very difficult to find sometimes, making the player spending the camera for a long time and not aware of the Phantoms.
Springtrap has five fingers on each hand, as opposed to the cartoonish four-fingered hands of the other animatronics. This is most likely because Springtrap's costume was (poorly) designed to fit a human inside.
Being the only animatronic who actually kills the player, Springtrap has two jumpscares.
This makes both Springtrap and Freddy Fazbear the only characters who have two jumpscares in their own individual games.
Springtrap, along with Bonnie, Mangle, The Puppet, and BB, are the only animatronics to lack eyebrows.
Despite being in the horror attraction while it burned to the ground, it is possible Springtrap survived. If one brightens the newspaper shown after completing Nightmare, Springtrap can be seen next to the Freddy figurine.
Springtrap is commonly theorized to be the original Bonnie model, as they both share a rabbit-like appearance, and Phone Guy refers to Springtrap as "Spring Bonnie".
Springtrap is also depicted to be one of the first two animatronics in the business along with Golden Freddy, the possible second hybrid (wearable costume and endoskeleton) animatronic, as Phone Guy states there were two, in the training tapes.
If the player is close to 6AM and Springtrap is at the doorway or window, it's best for the player to just sit there and wait because any movement of looking at the Monitor or Maintenance Panel will result in death.
Before having its name confirmed, the most common name for this animatronic coined by fans was 'Golden Bonnie' as fans believed that Springtrap was somehow connected to the similar looking animatronic Golden Freddy.
Springtrap's jumpscare is curiously the only jumpscare that does not result in an image of the player inside a costume.
Springtrap has only one single jumpscare in the Android version.