I've been reading a collection of Jean Borella's writings (translated) called The Secret of the Christian Way. (Borella does work in esoteric Christianity -- Christian gnosis [e.g., Clement of Alexandria], pseudognosis [i.e., what's usually called Gnosticism], responses to pseudognosis, etc.) He speaks at one point of Gnostic (in the heretical sense) aberrations, and notes a text, called Genna Marias (The Lineage of Mary), that claims that the Jews worshipped a man with the face of an ass who stood in the Holy of Holies, and that this was the reason why Zechariah was killed in the Temple. He notes that Flavius Josephus attributes a similar calumny to Apion (Against Apion Bk. II, 7), the claim that Jews put an ass's head in the Holy of Holies. Tacitus (Histories 1.5, 3, 4) also claims that the Jews are onolators. The Christians were also targets of this calumny; Tertullian deals with it in two works, Defense of the Christians agains the Gentiles and To the Nations.
JewishEncyclopedia.com gives an excellent summary of the history of this calumny. The source of it (as Borella also says) appears to be in Typhon worship: "Typhon" is the Greek name for Set, the murderer of Osiris, an underworld god.