The Physiologus was a predecessor of bestiaries (books of beasts). A bestiary is a medieval book that has short descriptions of various real or imaginary animals, birds and even rocks. ...

It consisted of descriptions of animals, birds, fantastic creatures; sometimes stones and plants, often with moral content. Some allegories set forth the deceptive enticements of the Devil and his defeat by Christ; others present qualities as examples to be imitated or avoided. Each animal was described, and an anecdote followed, together with the moral and symbolic qualities of the animal.

The Physiologus was compiled in Greek at Alexandria, perhaps for purposes of instruction, appeared probably in the second century, though some place its date at the end of the third or in the fourth century. In later centuries it was ascribed to various celebrated Fathers, especially St. Epiphanius, St. Basil, and St. Peter of Alexandria. Origen, however, had cited it under the title "Physiologus", while Clement of Alexandria and perhaps even Justin Martyr seem to have known it. It was translated into most Christian languages retaining its influence over people's minds in Europe for over a thousand years.