Here's some concept sketches I did for the lava monster in Missile Mouse: Rescue On Tankium3. When I'm writing out the story I'll often times sketch out designs on the same page. You can see here I was initially thinking he could be some sort of armored worm. Eventually he evolved into an armored insect/crustacean thing.
I compiled a few sketch pages into one below to show the progression of the design. I had a pretty good idea of what I wanted after the first couple sketches so a lot of these are roughly in the same ball park. I did lose the idea of him rolling up in a ball, though I still really like that idea.
And here's four pages from the book where we see this guy in action.