In some ancient civilizations the Wolf was a symbol of the underworld. In Egypt, the guardian of the dead was Anubis who was a wolf-headed god. The jackal portrayed as black was the symbol of both death and regeneration.
William is also the one who hid inside the suits and kidnapped and killed children, leading to the assumption that William Afton is The Purple Guy of FNaF 2 and 3 (he is killed at the end of the book when Charlie sets off the spring locks in his suit).
1 The code for Night 4's minigame in Five Nights at Freddy's 3 (395248) is the hex code of purple in reverse (#842593).  Many fans speculate that the child who cornered Purple Guy was the fifth child previously seen in the Give Gifts, Give Life Death Minigame in the second and Happiest Day minigame in the third game.