Bestiaries were an attempt to describe the natural world through a moral lens. Although the descriptions bear little resemblance to any real natural history, bestiaries are nonetheless an important artifact serving to cast light on the beliefs and superstitions of the medieval mind. It is important to note that bestiaries were not written as works of fiction or fantasy; the authors and their readers believed them to be true descriptions of unicorns, dragons and other bizarre and often grotesque monsters and beasts -- for just outside the narrow ambit of the walled village or hamlet, the world was an unsafe and frightening place. Men and women rarely traveled more than a few miles from their homes during their entire lifetimes, and so their knowledge of the outside world depended on the stories from stories and tall tales from travelers, that had been embellished by many retellings.
During the time period that bestiaries were most in vogue, most people in France and England lived in small fortified villages which were surrounded by dark forests. These forests supplied the inhabitants with firewood, herbs and venison, but at night the people retreated to their villages and abandoned the forests to their inhabitants, wild animals and men. Forests and their inhabitants were held in dread, and animal noises were attributed to demons and other creatures of the night. In such a pre-scientific world, in which the majority of people were uneducated and illiterate, it was easy to imagine that all sorts of incredible creatures lived only a short distance away. But since all animals were part of God's creation, the tendency was to regard them as part of God's plan - for this reason, animals were regarded not just as living creatures but as metaphors or symbols of divine lessons. In this sense, bestiaries shared a common view with Aesop's Fables in which animals such as the tortoise, the hare, the fox, and the wolf are used to convey moral lessons based on the stereotypical characteristics of these animals and their behaviours.
Bestiaries also influenced the decoration of illuminated manuscripts. Medieval illustrated manuscripts often featured animals, drawn from bestiaries, or scenes from Genesis in the margins.
Bestiaries died out with advancements in scientific knowledge, and as explorers opened up the unknown corners of the earth. However the tradition of bestiaries has enjoyed a sort of revival in modern times. The Argentine writer Jorge Luis Borges published The Book of Imaginary Beings, containing descriptions of many weird and wonderful animals from folklore and fiction. Bestiaries also continue to be a rich source of inspiration for fantasy writers and even games such as Dungeons and Dragons.
Our modern fascination with cryptozoology and unknown animals such as the Sasquatch or the Mokele-mbembe, a dinosaur supposedly still roaming unmapped parts of the Congo, suggests that our fascination with weird and wonderful animals and terrifying creatures living just beyond the pale of civilization have not disappeared in the harsh light of scientific knowledge. It is comforting to know that we have not conquered our world so completely that somewhere there may still be a dragon or two.
Perindens, a magic tree and keeper of the birds. From the Oxford Bestiary