From The Art and Popular Culture Encyclopedia
Drolleries (or drollery), often called a grotesque, are decorative thumbnail images in the margins of Illuminated manuscripts, most popular from about 1250 through the 15th century, though found earlier and later. The most common types of drollery images appear as mixed creatures, either between different animals, or between animals and human beings, or even between animals and plants or inorganic things. Examples include cocks with human heads, dogs carrying human masks, archers winding out of a fish’s mouth, bird-like dragons with an elephant’s head on the back. Often they have a thematic connection with the subject of the text of the page, and larger miniatures, and they usually form part of a wider scheme of decorated margins, though some are effectively doodles added later.
One manuscript, The Croy Hours, has so many it has become known as The Book of Drolleries.
Another manuscript that contains many drolleries is the Luttrell Psalter, which has hybrid creatures and other monsters on a great deal of the pages.