Quoting from H.W. Janson’s Apes and Ape Lore in the Middle Ages and the Renaissance:

The ape’s lack of tail had, of course, been noted in classical times, but it was not until the advent of Christianity that this member—or the absence of it—achieved metaphysical significance. Had not the Lord himself, according to Leviticus XXII, 23, declared the tail to be a necessary part of every animal by pronouncing those that lacked one unfit for sacrifice? …clearly, then, possession of a tail meant that the ‘end’ of its owner had been properly determined by the Lord, so that it was ‘against nature’ for any animal to be without one…according to a Talmudic tradition even Adam had at first been created with a tail, but God had removed it, for one reason or another.

Thus the Physiologus could view the ape’s unadorned posterior both as an indication of hybris, of the animal’s desire to ‘rise above his station’ and to be like man, and as proof of Divine disfavor. The area in question, with its ischial callosities and raw patches of sexual skin, certainly deserved to be called turpis (the Greeks scathingly characterised it as ἄπνγος); it was only natural, therefore, to conclude that the ape must have had a tail originally, since God had created all things perfect, and that he was deprived of it when he ‘fell from grace.’