The name "Pyewacket" comes from the 1647 British pamphlet "The Discovery of Witches," in which author Matthew Hopkins claims to have interrogated witches in Essex County. One witch names many familiars, including the spirit Pyewacket which appeared to her as some sort of animal.
When the milk is spilled in the fridge, its shape resembles the film's poster.
Pyewacket was the name of Gillian's cat, who was also Gillian's familiar assisting in her spell casting in Bell Book and Candle (1958).