Augustine of Hippo
Augustine was born at Thagaste, in proconsular Numidia, in 354; he died at Hippo Regius in 430 (both places are in the present Algeria, near Tunis, the ancient Carthage). His father was an influential member of the council, but the family was not rich. His mother was Christian, though his father was not until late in life. Augustine was first educated at Thagaste, where he learned to read and write and was exposed to the rudiments of Greek and Latin literature. Later his father sent him first to the neighboring Madaura, and then to Carthage, some two days' journey away, for additional education. There he became a Manichean, a sect considered heretical by the Catholic church, that was very popular at the time. He remained a member of the sect for nine years. He moved to Rome around 383, and from there to Milan. His conviction of the truth of Manichean doctrine failing, he turned first to Neo-Platonism, and later, after a move to Milan, where he came under the influence of Ambrose, he converted to Christianity. He returned to North Africa around 388, where he was ordained and eventually became bishop of Hippo.
Augustine was a prolific writer, and his books had a great and lasting influence on the doctrine of the Church. His works were read throughout the Middle Ages and beyond. Two texts were particularly popular: the Confessions, a sort of penitential autobiography; and The City of God (De civitate Dei), a treatise on and a defense of the doctrines of the early Church. It is this last work that is of interest here; in it Augustine uses some of the commonly-accepted beliefs of the time about animals to support his arguments. He also discusses the monstrous races of humans, giving the reports of their existence cautious acceptance; though he says "we are not bound to believe all we hear of these monstrosities" he then goes on to say "to conclude this question cautiously and guardedly, either these things which have been told of some races have no existence at all; or if they do exist, they are not human races; or if they are human, they are descended from Adam."
With the respect Augustine had in the Middle Ages, his opinions were taken seriously. His acceptance, though cautious, of some of the beast tales could only have made them more acceptable to later writers.
Relevant excerpts from The City of God can be seen here; click the "Texts" tab to read them.