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).
That leaves us with Mike Schmidt, the security guard from FNAF and Fritz Smith, the security guard from the custom night of FNAF 2. Unfortunately, I can't find any evidence that proves that one of them is Purple Guy so we will have to do some detective work. After some thought, my money is on Fritz Smith.