“Dragon” is a catchall word used to describe several dozen species of large avians that are all descended from a single prehistoric ancestor, Pteroignis. All of them are intelligent, though not all of them are mages. All dragons possess wings (though some are flightless), feathers (in the form of either true feathers or “scales”), and are warm-blooded. They are generally divided into three categories for easy classification: Alkies, thanes, and cold dragons.
Alkies are dragons who produce fire via a series of alcohol-producing glands. Yeast and bacteria in the glands guide the fermentation process. If they use up all their alcohol to breathe fire, it can take anywhere from a few days to a month for them to fully refill.
Notable species of alkies include the European Lindworm.
Thanes are dragons who produce fire by igniting methane gas. Naturally-occurring bacteria in their stomachs produce excess methane, which they can ignite. However, they cannot breathe fire unless they have eaten recently, as they have no mechanism by which to store the gas for long periods of time.
Cold dragons are dragons that cannot breathe fire.
The Arowana is one of the major species of cold dragon.
The ancient ancestor of all modern dragons, Pteroignis, is believed to have branched off from Archaeopteryx at a point when a mutation caused Archaeopteryx to sprout four wings instead of two. (Today, this mutation is still known to occur occasionally in chickens.) Using its extra wings as a set of clumsy forelimbs, the forebearer still to Pteroignis eventually regained its front legs while yet retaining its wings. Today, dragons are unique among all vertebrate species in this aspect. However, it was some time yet before dragons developed the ability to fly--their wings had to grow larger, and their bones hollow, before this was feasible.
It is only recently, geologically speaking, that dragons have come to have "scales." Dragon scales are actually an adapted form of feather, which is both stiff and lightweight. Examining a dragon scale up close reveals the remnants of a shaft running down its center, and its thin, feather-like shape. Many dragon species, of course, still have feathers in addition to drakescales--notably, the South American Crested, with its large, impressive, cockatoo-like crest. Many dragons have horns as well, also adapted from feathers at some point in the past. (A few dragons even have beaks, such as the Lesser Caribbean, but they are uncommon.)
One thing about dragons that has remained reletively unchanged since their genesis is their diet. Dragons are primarily fish-eaters, and they have long, needle-like teeth for the purpose of hunting fish.