Got Medieval has the market sewn up when it comes to manuscript marginalia and all the beasts and creatures that lurk therein. But another good resources is Medieval Bestiary, which has an alphabetical lists of the animals that occur in medieval texts and their attributes in the medieval mind. Many bestiaries survive from the middle ages: they normally consisted of lists and illustrations of animals (both real and mythological) along with any related moral lessons. For example, the donkey was slow and resisted commands, while when it comes to apes “the female ape always gives birth to twins, one of which she loves and the other she hates”.
The site also does a good job of showing what ancient sources bestiaries drew on, which shows a certain intellectual continuity between the Classical world and the medieval. There are plenty of examples of the associated manuscript illustrations too, which are nice in themselves.