Hana Videen at Lit Hub:
Although bestiaries were popular texts in medieval Europe, many of their tales derive from a far older text from northern Africa known as the Physiologus. The Physiologus (meaning Natural Philosopher) was originally written in Greek by an unknown author, probably someone living in Alexandria during the third century CE. This text in turn is made up of stories whose influences can be traced even further back in time to texts on natural philosophy and religion by ancient Greek and Roman writers.
As the Physiologus further developed over the centuries, its age-old tales were often shaped by contemporary authorities like the third-century geographer and grammarian Solinus, or Ambrose, a fourth-century bishop and theologian. So while the ones of a story might stay the same, the interpretation and moral might shift according to the ideas that pleased the contemporary scribe.
more here.