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).
Both characters from FNAF 3 cannot be the Purple Guy because they are alive a couple of decades after Purple Guy's death and the security guard has been with Spingtrap. Due to Game Theory's FNAF 2 video, there is a lot of popularity towards the Phone Guy being the Purple Guy.