Scribes decorated medieval manuscripts with all kinds of weird thing, like rabbits, cats, Star Wars characters, and lots of snails. Snails very often appear to be doing battles with knights -or, more accurately, the knight is attempting to do battle with the snail. Vox takes a closer look at this phenomena.
It could have been a political insult at the beginning, which turned into an inside joke over time. A medieval meme, as it were. -Thanks, Phil Edwards!