About the AU
Spawned from fan-art, mock-up art, and at least one fan video, Five Nights at Freddy's Au is based on the popular online video game series Five Nights at Freddy's which collected a very loyal fan base since the first installment of the series.
Because of the popularity of The Big Four and their contrast in personalities, they have been connected to the 4 staring animatronics in the series. Some have even connected them because of certain colors of the character and the animatronic.
The Big Four
Older Hiccup is usually placed in the role of Bonnie. Being that Bonnie plays the guitar, people have proposed that Bonnie is quite awkward and sensitive in nature, which fits nicely for Hiccup as he comes of age. Others have speculated since Bonnie is a rabbit, Hiccup's outdoor setting fits him for the role of Bonnie as well.
Is seen normally as Freddy Fazbear. Fans of Five Nights see Freddy as a sort of leader of the group, since he is the lead singer for the band, as Jack Frost is in the Big Four. Although physically, Freddy is similar to Jack Frost by his blue eyes.
She often plays Foxy. Most have connected her to the crimson animatronic because of her wild red hair. Others have connected her to Foxy through her wild personality, which is much like a fox, and by the fact that Foxy is an outcast animatronic, which would relate to Merida's individuality and being the only one with a accent in the group.
She is commonly thought to be Chica. Most consider her for the role because of her long blonde hair which relates to Chica's golden color. Since Chica is speculated to be the only female within the four animatronics, people place her in this role because of her similar girly nature.
Most people actually make her own animatronic because of her troubled backstory and also since the creators of Frozen admitted that they planned to make her evil like the Snow Queen, but later scrapped that idea.