Five Nights at Freddy's minis are mini-Animatronics.
The haunted animatronics of Fazbear Entertainment are the primary antagonists of the Five Nights series. There are four basic animatronic molds that most others spring from: Freddy Fazbear, Bonnie the rabbit, Chica the chicken, and Foxy the pirate fox. In addition, there also exists the Puppet and BB/Balloon Boy.
The minis take on a variety of shapes. If the misspelled character was an animatronic, it resembles that character in the game the fic is set in; for instance, misspelling Freddy's name in a Five Nights at Freddy's 2-era fic would result in a mini-Toy Freddy.
For misspellings not associated with a specific animatronic (such as the names of the player characters), a mini-Hallucination is created, again dependent on which game the fic is set in (mini-Golden Freddys for Five Nights 1, mini-Shadow Bonnies for 2, mini-Shadow Freddys for 3, and mini-Nightmares for 4).