Among the Biblical scenes and religious iconography of the Middle Ages, there are some rather fantastical creatures on the pages of illuminated manuscripts and in church windows. The juxtaposition of saints with winged serpents seems contradictory, and art historians have yet to come up with a concrete explanation for their popularity and inclusion. These seemingly secular tidbits have no literal connection to the scenes they were meant to offset, making their existence all the more puzzling.
Some think they were light-hearted diversions from the imaginations of bored monks or window makers, but this seems to be an oversimplification that doesn’t account for their widespread presence. Most people were illiterate and associated religion with visuals more than text, so every detail of medieval art seems highly relevant. On the other hand, taking a more analytical approach and searching for hidden symbolism and social commentary is fruitless, because there is no evidence for how the medieval audience had reacted to hybrid creatures and monsters. Instead, the art historian Andrew Otwell suggests that “they are not meaningless, or merely entertaining, but by themselves they do not contain complete meaning.”
More about stained glass creatures: http://www.vidimus.org/panelOfMonths.html
More about medieval marginalia: http://www.heyotwell.com/work/arthistory/marginalia.html
(stained glass from Christ Church Cathedral in Oxford)