I remember reading an article on snail combats in the medievalist journal Speculum, back when I was in college. These combats just started appearing in the margins, then they went away, 100 or so years later, with no explanation.
Nobody knows what it means. It's more or less contemporary with the expansion of glaciers in the Little Ice Age, but that was an actual event with real documentation (even if the documentation for the glaciers is spotty, and not as thorough as it might be). If some race of giant snails appeared at this time, you'd definitely think there would be accounts of battles, notes of some important lord's death at the hands(?) of the snails, or something more than marginalia.
Those shells would seem to give the snails a claim to be the knights of their gastropod world. Wikipedia also points out that some species of snails use "a venomous harpoon" to catch faster moving prey, like fishes.
I did once see a movie, perhaps called Gargoyles, where the gargoyles hatched in batches roughly 700 years apart. Perhaps the humans defeated the snails, but a new generation of giant snails can be expected one of these days.
In any case, to the best of my knowledge, no miniature manufacturers actually make giant snails--probably a lost opportunity.
Grelber
Tango, thanks for posting this. As you can see, I read an article on this subject once, years ago, and it stuck with me, and I've even thought about it from time to time. Haven't gone anywhere with it, or pitted my knights against snails, but I do think about it.