That is a fine example of marginalia, which have become somewhat famous because of the often ludicrous scenes they show us. Other famous marginalia show rabbits jousting on snails, fart trumpets and even people harvesting penis-bushes. You can read about them here and here.

The modern ability to isolate these often outlandish and obscene images from their texts and circulate them without their context online often emphasises their unusualness, but they're not simply surreal cartoons, and for every snail-man hybrid, there are a dozen perfectly ordinary illustrations that receive no attention. Often the illustrations involved some sort of pun, reference or allegory to the text in question. A section of text which may warn of an upset of the natural order of things, for example, could well be illustrated by a hare hunting a dog, as a visual emphasis on just how bad things could become. It's also possible that these served as little in-jokes, or a gentle jibe between hard-working scribes - who were essentially professional craftsmen - and their largely noble audiences.