D&D Monsters: Carrion Crawlers and Purple Worms

The majority of giant invertebrate monsters in the early editions of D&D are, at least in general terms, arthropods - the group of jointed-limbed creatures with chitinous exoskeletons to which insects, spiders, and crabs belong, among others. Exceptions include giant octopuses and giant squid, which are at least partially based on real animals, a few aberrations that are difficult to ally with anything real, and, somewhere in between, the purple worm. As originally drawn, the carrion crawler also qualifies, although this changes in later editions.

The worm-like body plan is a common one among real-world invertebrates, being found in a wide range of creatures that aren't all that closely related. But, on the whole, they aren't very fearsome, and the different types don't have the recognisability factor that spiders, scorpions, praying mantises, or whatever, might have. Even in D&D, the primary threat from the purple worm comes from the fact that it's just really big. The carrion crawler is a different matter, but, then it is rather less wormlike. Both creatures have 'Ronseal' style names, although, at least to my mind, 'carrion crawler' is, like 'mind flayer', one of the more evocative ones.

D&D Monsters: Rust Monsters

The rust monster is, like the bulette, a creature inspired by a cheap plastic toy bought in Hong Kong. The original toy apparently looked like a lobster with a propeller for a tail, perhaps to represent some weird aquatic kaiju. Gygax later admitted that he'd initially been stumped as to what special attack to give such a ridiculous-looking creature, but, of course, he eventually came up with something that made it a particularly memorable and unique monster. One that is essentially a walking hazard, an annoyance that's completely harmless by the usual standards of the game but nonetheless very much to be feared. 

Albeit, like displacer beasts and piercers, one whose name is just a rather bland description of what it does.