The British Library posted a wonderful group of images of marginalia featuring knights facing off against snails, almost all from the 14th century.
Knight v Snail II: Battle in the Margins (from the Gorleston Psalter, England (Suffolk), 1310-1324, Add MS 49622, f. 193v. More at Knight v Snail.
Got Medieval also touched on this subject some time before, but with fewer illustrations, and at pre-Gébelin Tarot History, Michael Hurst explored it (mainly about halfway down).
Smithsonian also took it up, gathering a couple of these and other references, and Homo Ludditus expanded on the subject, adding a few more. Strange and wonderful.