Oh the horrors that giant molten rock shell hides. Want an idea of how snails eat? Just run a search on youtube, carnivorous snails are kinda scary, glad their too small to eat us! Unlike this big fella.
Why hasn't this happened yet? I mean, if it's a matter of speed, maybe give them little legs or make them slither like snakes, but I think having an armored enemy skilled at projectile attacks (farts, spitting acid slime) would make for an excellent monster, large or small.
It'd do one seriously fast charge attack with it crawling into its shell and then the shell rolls lik sonic into a wall where it gets stuck and its underside would be vulnerable for a few seconds... Thats when we attack >o<)b