Forms of evil
As is shown here, the devil may take on different guises in the illuminations we find in these manuscripts, but there are three basic ingredients: man, animal, and pure phantasy. When he tries to tempt Christ in the desert, or when he fools a nobleman, he looks almost human. But the miniaturist makes sure he does not fool us: his feet are that of an animal, or his eyes are burning coals.
The pictures tell us, that he may turn himself into a cat. He may also be a pale, threatening creature taking souls to hell. The colourful monsters pestering saint Pachomius could have been designed by Bosch. Finally, the devil that seduces Theophilus to sell his soul, looks like one of the more malicious Muppets designed by Jim Henson.