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.