D&D Monsters: Red Dragons

Many medieval descriptions of dragons make no mention of them breathing fire, and this does not seem to be part of the original myths of the creatures. Nonetheless, the idea that dragons breathe flame does seem to have originated in medieval Europe, and is now de rigueur in fantasy depictions. It probably arose because of the association of evil dragons with hellfire, and the general idea of fire-breathing creatures certainly predates it (Leviathan breathes fire in The Book of Job, for example). 

In D&D, of course, it was originally decided that the five types of chromatic dragon should be distinguished by each having a unique attack, so that green dragons breathe poison, black dragons acid, and so on. Naturally, the most powerful of all the chromatic dragons was going to be the one that breathed fire, fitting the legends on which the broader idea is based.

D&D Monsters: Green Dragons

Medieval descriptions of dragons imply that they look rather like snakes, but they were said not to be venomous and, since they weren't always fire-breathing either, were often just very very strong. Some medieval bestiaries, for instance, describe dragons killing elephants by wrapping them up in their tail and squeezing like a constrictor. The poisonous counterpart of the dragon in medieval lore is the basilisk - which is basically just an incredibly deadly snake, much like the version in The Philosopher's Stone.

In D&D, the basilisk is quite a different creature, and very far from being legless. While the association of poison with serpentine beings make sense, it's not common in depictions of dragons. The D&D idea of certain dragons belching poisonous gas instead of something flammable is likely an original one - something added so that each of the five chromatic dragons had a unique attack mode. And in this case, of course, that's the green dragon, the mid-point in the five-point scale of increasing chromatic dragon power.

D&D Monsters: Black Dragons

It's hard to argue with the notion that the single most iconic monster in fantasy fiction is the dragon. Dragons exist in some of the oldest myths known and are found across many different cultures. Having said which, this only holds true for a sufficiently broad definition of the word, since there isn't really much in common between, say, European and Chinese dragons beyond the fact that they both have snake-like bodies with legs. Even that is less true today; most modern depictions of western dragons aren't as serpentine as those drawn in the Middle Ages usually were.

Given that they're right there in the name of the game, dragons are obviously fairly key to D&D. In the 1st edition, they receive more detailed options than other monsters, having eight age categories and three size classes, and a suite of special abilities right from the beginning. Furthermore, there are no less than ten different kinds of true dragon, divided evenly between the good 'metallic' and the evil 'chromatic' species.

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.

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. 

D&D Monsters: Remorhaz(es)

The remorhaz is another creature original to D&D, and one of the few such that is honoured with a proper name rather than a descriptive term. It's one of the more powerful creatures in the 1st edition that are described as simply animals, rather than intelligent beings. This is largely because it's so difficult to hurt, although it also has a bite that does more damage than that of most dragons. In later editions, it's toned down, although it remains quite fearsome.

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1E

As originally depicted in the 1E Monster Manual, the remorhaz has a snake-like body, with a thick leathery hide that's harder to penetrate than plate steel. The upper surface of the body is covered with an irregular array of white protuberances, which apparently shed excess heat from the intestines. The lower surface has a series of wide flat scales, which also resemble those seen on many snakes, reinforcing the resemblance. The key difference, of course, is that the remorhaz also has a huge number of legs. These are multi-jointed and covered with an exoskeleton; the segments within the leg are all of similar size, suggesting the relatively simple structure of a centipede's legs rather than those of a spider or insect.