Bestiaries were particularly popular in England and France around the 12th century and were mainly compilations of earlier texts, especially the Physiologus and the Etymologiae of Isidore of Seville.
One important part of most bestiaries was the illustrations. They added a lot to the descriptions, serving then as an educational tool for the illiterate.
The most well-known bestiary of that time is the Aberdeen Bestiary. There are many others; over 50 manuscripts survive today.
T.H. White's translation of a medieval bestiary can be found on-line at " class="external">http://libtext.library.wisc.edu/Bestiary/
More information on the Bestiary can be found at The Medieval Bestiary.
Two other online bestiaries can be found at the National Library of Denmark website: The Bestiaire of Philippe de Thaon and The Bestiary of Anne Walshe.