The Medieval Bestiary
Here begins the book of the nature of beasts.
Of lions and panthers and tigers,
wolves and foxes, dogs and apes.
~ Aberdeen Bestiary

I seldom recommend whole sites in this page, but this one deserves a careful exploration. You will find detailed descriptions of several manuscripts, a list of the beasts (many fantastic but all with fantastic descriptions) with lots of pictures and a bibliography. If this is not enough, you can download pdf editions of old texts about bestiaries.