An Auto Automaton was a perfect robotic replica of a Mobian, originally created by Dr. Ivo Robotnik. Auto Automatons were designed to infiltrate various Freedom Fighter groups and gather information. More advanced versions of these robots were known as Infiltrators.
A series of androids created by Dr. Robotnik, the Auto Automatons were first designed for espionage purposes. The Auto Automatons were intended to be exact duplicates of the Freedom Fighters, thus enabling them to replace their biological counterparts. The first functioning Auto Automaton model was based off Sally Acorn and was used to deceive Royal Underground member Geoffrey St. John. The Auto Automaton of Sally was discovered to be a robot however and destroyed with help from Sally's Trainees. The factory which was mass producing Auto Automatons was also destroyed. (PSM: #1, #2, #3)
Another of the few surviving Auto Automatons, based on Fiona Fox, was used to deceive Tails during one of his early solo adventures. Created several years before Robotnik put it to use, it still had the appearance of a younger Fiona. Tails fell in love with the replica Fiona, and was heart broken when he discovered it to be a robot. However, Tails would still not fight the Auto Automaton, and only survived because they fell into the water where the robot rusted. Tails left the robot on the island, briefly expressing hope that he could repair her, but giving up upon Robotnik's retreat. Tails soon left, and never saw the robot shed tears at the sight of him leaving. Despite acting on Robotnik's orders, the robotic Fiona appears to have genuinely cared for Tails. Unfortunately, her attachment led her to want him subjected to roboticization so they could be together. (StH: #28, #29)
While the Freedom Fighters left Knothole to obtain the Sword of Acorns, which had been obtained by Knuckles the Echidna, King Max was captured and replaced with an Auto Automaton of himself, which deceived everyone until Robotnik exposed it to be a fake after invading Knothole. It's mission accomplished, Robotnik callously destroyed his creation. The Auto Automatons were eventually discarded as faulty, given that the materials that made them so lifelike made them so susceptible to rust that they would become useless within seconds of touching water. (StH: #47, #48, #49, SSS: #1)
When Mighty the Armadillo, Nic the Weasel and the real Fiona Fox returned to one of Robotnik's forced labor camps (where Mighty, Fiona and Ray the Flying Squirrel had been held captive as children), they found old, unused Auto Automatons based on the Freedom Fighters. (KtE: #29)
Years later, Dr. Eggman created similar but superior robots, instead calling them Infiltrators. An Auto Automaton/Infiltrator of Tommy Turtle was created and nearly killed Sonic after badly damaging Sonic's father. After this Auto Automaton's destruction, Sonic and the Freedom Fighters went to New Megaopolis where they found thousands of Auto Automatons being mass produced. The factory was subsequently destroyed and the real Tommy was rescued. (StH: #135, #136, #137)
Some time later, Eggman tried again, this time using Renfield T. Rodent's Casinopolis in Station Square. The casino had scanners in slot machines that would copy individual data, which would then be used to make infiltrators to replace the originals. The Chaotix were chosen by the President to investigate as they'd had previous experience with Renfield. Renfield managed to use an Infiltrator copy of a Mobian horse to capture Knuckles, and ensnared Vector the Crocodile, Espio the Chameleon, and Rouge the Bat with a scanner disguised as a slot machine. Despite being discovered, Renfield gloated that the amount of data already collected would let him build a small army. However, Julie-Su unplugged the computer, destroying the transferred data and preventing it from being used, and Mighty the Armadillo destroyed the factory, saying no one could use an army of robots that had not been created yet. Renfield was subsequently arrested and tried in Station Square courts. (StH: #166)
In other media
Sonic the Hedgehog (TV series)
Similar to the Auto Automatons was an android created by Robotnik in the Sonic the Hedgehog cartoon. This android, named Sallybot, was created to resemble Sally down to the finest detail, and was also programmed with a copy of her personality. However, lacking her true memories, the pseudo-Sally was unable to fool Tails when she didn't know a story to read and do the "scary witch voice". She was then exposed later during another reconnaissance mission, and reprogrammed by the Freedom Fighters to serve their purposes. As a final act, Sonic replaced the real Sally with the fake, causing the Roboticizer to overload. Sally's robotic double from the TV series was also able to fire bolts of energy and a sludge-like substance known as "Mega Muck" (which is mentioned by Uncle Chuck as being roughly equal parts petroleum, quicksand, and clay mud) from her fingertips.