D&D Monsters: Metallic Dragons

In European myth, dragons are almost universally regarded as evil - rapacious monsters that lay waste to the countryside, have to be slain by heroic knights and, in many cases, essentially breathe hellfire. They are, sometimes literally, diabolic creatures. The same is not necessarily true in other cultures, such as that of China, where dragons may not necessarily be easy to parlay with or recruit as allies, but aren't fundamentally hostile, either.

In D&D, the good counterparts to the evil chromatic dragons are, of course, the metallic ones. Indeed, they are among a relatively small number of 'good monsters' to make it consistently through into the core books of later editions. Up until 5E, they are portrayed as rarer, but individually more powerful than, the chromatic dragons. They are perhaps even rarer in games than they are usually described as being in the universe (in two campaigns of Critical Role, the PCs have so far encountered at least seven chromatic dragons, and only one metallic). Doubtless, this is because they are less useful in a typical game if you're not going to fight them - and they're too powerful to be regular allies.

D&D Monsters: Blue Dragons

A considerable number of mythological deities are said to throw thunderbolts - Zeus and Thor are merely the most familiar of these to Europeans, with examples known from many other cultures. Actual mythic creatures that throw lightning, however, are much less common, although Chinese dragons are at least associated with thunder and storms. In D&D, however, it seems an obvious attack mode once we've dealt with fire and ice, and, naturally enough, it's associated with the dragon that's the colour of the sky.

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1E

The original picture of the blue dragon shows an animal with a moderately long and heavy snout, large eyes, and elongated canine teeth. More distinctive features include the fact that the dorsal frill of the green and black dragons is here replaced with a series of large triangular plates, which may be projections from the vertebrae, but could be separate structures similar to those on a Stegosaurus. The latter possibility is supported by the existence of three similar plates on the forehead. In front of these, there is a row of four  bony spikes, one of which is enlarged to form a narrow nasal horn. As usual, this doesn't look like it would be of much use in combat, at least compared to the teeth and claws, so it might be a display structure.

D&D Monsters: White Dragons

While witches, evil sorcerers, and the like, may create freezing cold storms, beings that attack by virtue of simply being very cold are not common in myth, or indeed, in early fantasy literature. Tolkien mentions "cold drakes", but these are simply dragons that don't breathe fire, rather than being any supernaturally low temperature. Nonetheless, when Gygax was looking around for different attacks for the five basic chromatic dragons, intense cold seems (at least in retrospect) an obvious fit. Perhaps as a counterpoint to the fiery dragons being the most powerful these, the white dragons, became the weakest.

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1E

White dragons are not only the weakest of the chromatic dragons but the weakest of dragons overall. A step down from the black dragons, while they do have much thicker hides and stronger jaws, 1E white dragons have about the same ability to sustain physical injury as a tiger. In fact, their claws do less damage than a tiger's do, so it's possible that their legs are actually less muscular (or the claws are blunter, or smaller, which seems unlikely).

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.