Sometimes the knight vs. snail pictures are weirdly unfinished, like they were scribbled in as an afterthought. It's almost as if this bizarre clash between man and mollusk was the first thing every illustrator thought of when they were mindlessly doodling. It was the 13th-century version of dickbutt.
via British Library
"'Tis what m'lady doth professed."
Oddly enough, for an image this popular, there are apparently no written records to explain why knights vs. snails were considered such a hot idea. It's a "great unsolved mystery of medieval manuscripts," like how the ones written on human flesh always bite for your fingers.
The first person who theorized about epic snail-on-knight combat, all the way back in 1850, was the wonderfully named Comte de Bastard, who said it might be a symbol of the Resurrection. But this was based on him seeing only two images, both of which were near illustrations of Lazarus rising from the grave. So he might have been taking wild guesses based on the last thing he saw. A more accepted theory is that the slimy snails might be a symbol for Lombards, a group widely loathed for backstabbing, lending money at interest, and "non-chivalrous comportment in general." But that doesn't explain why the knight often looks shocked or terrified at seeing a gigantic snail monster.
With so little to go on, medieval scholars floated pretty much every possible explanation. For example, that it's a representation of medieval class struggle, or an exaggerated depiction of fighting pesky garden pests. Or maybe it's a dumb joke doodled for fun by bored illustrators and we're supposed to laugh at the knight for cowering before such a "heavily armored" opponent. And naturally, at one point somebody floated the theory that the snail is somehow a symbol of female sexuality -- something also known for being both impenetrable and moist.