I'm currently sculpting a couple snails and a scorpion like nasty thing.
Because all my projects start with some concept sketch here are some studies I did a while back for my Snails of war.
You may think giant snails in D&D are a bit lame visually but let me tell you that I did find them quite menacing. Here are the variants shells that I enjoyed the most.
Lets hope they translate well into sculpts.