D&D Monsters: Wyverns

Actual medieval descriptions of dragons were vague as to whether they had four legs, plus wings, or just two, and illustrations of the time were similarly variable. It only seems to be around the 16th century that the word 'wyvern' (which had previously meant 'viper') was used to specifically mean a two-legged dragon, and possibly only so that heralds knew how many legs they were supposed to be drawing on coats-of-arms that previously only mentioned a 'dragon'. There was no indication at the time that there was any other difference between wyverns and what we'd now think of as true dragons.

When Gygax adopted the term for D&D, he made wyverns somewhat weaker than true dragons, and unable to breathe fire. Traditional illustrations often show two-legged dragons with snake-like tails ending in a sharp point, and this became the poison stinger seen in D&D. Perhaps following on from this, there has been a tendency in fantasy fiction to make wyverns weaker than four-legged dragons, although that's perhaps turning round again more recently - in both the books and TV series, for instance, the fire-breathing and deadly dragons of Game of Thrones only have two legs.

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.