A bestiary is a type of book, common in the medieval era, that catalogs animals. Medieval bestiaries typically included both real and mythological creatures, thereby incorporating natural history, cryptozoology, and legend. Plants and minerals were also sometimes included in bestiaries.
A typical bestiary entry includes a drawing of the animal, a description of its attributes and habits, and a related moral lesson for readers. The symbolic associations of various animals represented in bestiaries are an integral part of cultural symbolism in the West. For example, even today many people have internalized the symbolism of a lion standing for courage or a donkey for stubbornness.
Though medieval examples are the best known, the bestiary first appeared in the early Christian era in Greece. The Physiologus, written anonymously in 2nd century Greece, made connections between classical Greek scholarship on natural history and Christian moral lessons. Staples of the bestiary genre, such as the pelican who tears her own breast to feed her young and the phoenix that rises from its own funeral pyre, first appeared in the Physiologus.
Other early examples of the bestiary were compiled by Saint Ambrose and Saint Isidore of Seville. Two of the most well known bestiaries of the medieval era are the Aberdeen Bestiary and the Ashmole Bestiary, both dating from around the 12th century. Many early and medieval bestiaries are preserved in libraries and museums, and some are on display to the public. They often include stunning illustrations or illumination.
The idea of the bestiary also survived past the medieval era, and modern examples can often be found. French illustrator Henri de Toulouse-Lautrec and Argentine author Jorge Luis Borges are two more recent authors of bestiaries. Reference bestiaries are often available for fantasy games such as Dungeons and Dragons. If you are curious about what a medieval bestiary looked like, there is an interesting, interactive website featuring material from various medieval sources at bestiary.ca.