A book about fantastic creatures found in Newt Scamander’s shed inside his brown leather suitcase (WFT).
Bestiaries of the Medieval era were compendiums of fact, legend, and lore about both real and mythological creatures. The fact that Newt Scamander owned a copy reveals that his own book Fantastic Beasts and Where to Find Them was written in the same historical context with many of the same creatures, and meant to be just as inspirational to the readership.
. . . Bestiaries were made popular in the Middle Ages in illustrated volumes that described various animals, birds and even rocks. The natural history and illustration of each beast was usually accompanied by a moral lesson. This reflected the belief that the world itself was the Word of God, and that every living thing had its own special meaning. ....
. . . Bestiaries were organized in different ways based upon the text. The descriptions could be organized by animal groupings, such as terrestrial and marine creatures, or presented in an alphabetical manner. However, the texts gave no distinction between existing and imaginary animals. Descriptions of creatures such as dragons, unicorns, basilisk, griffin and caladrius were common in such works and found intermingled amongst accounts of bears, boars, deer, lions, and elephants.
From the Web
The Medieval Bestiary: Animals in the Middle Ages
Online example of a Medieval Bestiary from the British Library