When i first moved to West Australia, I stumbled across an adult one of these things (they are more grey with harder shell), it looked like a Sandworm from DUne. I thought it was from outer space
called a Sandgroper ( FYI, nickname for West Aussies is Sandgroper)
About 3-4 inches long the one I found, would make a perfect miniature for a Thunderherder
Ever since I saw it on one of the BBC's David Attenborough wildife series (Life in the Undergrowth, I think) I always thought the Velvet Worm, sized up, would make an excellent D&D monster, what with its general appearance, sensory antennae, and its ability to incapacitate foes by squirting silk-based glue over them.