The Fox is a minor antagonist in Ultimate Custom Night. He's the anime counterpart of Foxy the Pirate.
The Fox is the main antagonist of A Journey of Revenge. During the various segments that composes the anime, The Bear constantly tries to enter into the temple with the intent of attacking killing The Fox, in an attempt to achieve revenge on him for something unknown he did in the past. However, The Fox always understands his plan, and he defeats him every time. Then, The Fox humiliates The Bear with weird punishments for his behavior towards him.
It's unknown if the anime cutscenes have something to do with the main story of Five Nights at Freddy's. However many thinks The Fox represents William Afton, the main antagonist of the saga, and The Bear represents Henry Emily, who tries to find and kill him to avenge the homicide of his daughter Charlie. Another possibility is that the Fox is a metaphore for the big brother, who dresses as a fox for the entirety of Five Nights at Freddy's 4 and The Bear the bite victim, who was constantly bullied by his older brother, and he could be trying to kill him in a spiritual form to avenge his murder.