Dragons

Dragons are superficially-reptilian creatures found all over the Known World. Approximately one and a half times the size of a horse, they have four legs and two wings, and a long neck and tail. Their bodies are covered in scales - individual scales have variations in tone, brightness and sheen and can form patterns or just a random distribution, but each dragon is a single overall color. Dragons are born with transparent scales which gradually adopt a color as they mature - it is not known what causes the color selection, but there does not appear to be a hereditary component. Dragons appear in almost any color.
Dragons are capable of projecting offensive materials - most commonly flame, but also acid, poison, smoke, even blasts of cold or lightning bolts - from their mouths. This, like much of dragon physiology, is not understood and is considered to be entirely magical in nature. The color of a dragon's scales usually - but not always - has a correlation with the kind of "breath" is has - red dragons breathe fire, green dragons belch poison, blue dragons spit lightning bolts etc.
Dragons are semi-intelligent - more intelligent than a dog and perhaps as intelligent as an ape. They are territorial carnivorous hunters and cannot be tamed (although some people have kept them as exotic pets in a menagerie or even used them as guard animals). Dragons can fly and are formidable hunters, with excellent senses and reactions. Their scales provide a degree of armor.
Dragons are commonly said to hatch from eggs, but this is not entirely accurate. A dragon "egg" is actually a dragon fetus tightly curled around a nutrient sac. When "laid" (really, born) the "egg" is the size of a hen's egg but quickly grows. The fetus initially feeds from the nutrient sac, but soon supplements itself with a soup of pre-digested food regurgitated by the mother. A clutch of "eggs" is lain in a rocky hollow filled with regurgitated food - stomach acid also dissolves the rock and provides much-needed minerals for the infant dragon's bones and scales.
The "egg" takes the appearance of an elongated sphere covered in scales - the scales of the dragon's back and flanks. Over time, these scales thicken and darken through exposure to acid and the air, becoming glossy black or dark brown after a few months. Gestation of dragons is very slow, taking approximately seven years for the "egg" to reach about eight inches long.
At the end of this period, the "egg" "hatches" - in truth, the dragon unfolds itself and molts (sheds its skin) for the first time. The outer layers of skin and scales have come fused together with acid and dirt, and so the process looks very much like the hatching of a reptile or bird. The infant dragon that emerges is about eighteen inches long and has transparent scales, with its flesh and some blood vessels visible through them.
Dragons eat meat from "birth" but their small size means they continue to eat regurgitated food as well as worms, birds and pre-chewed flesh from larger meals. While dragon mothers nurture their children (dragon fathers play no role in raising their offspring) until they are mature (approximately a decade later) an infant dragon can fend for itself by hunting worms, frogs and small rodents.
A dragon is physically mature around ten years after "hatching" and has grown to about one and half the size of a horse. Its scales will have developed a color over this period, and its "breath" will appear when it is physically mature. A dragon is capable of growing larger than this if able to obtain sufficient food and a maintain a large enough territory - but few do owning to population pressures and habitat restrictions caused by human civilization.
Knowledge of the lifespan of wild dragons is poor; dragons living near human populations (which compete with humans for prey species or prey on livestock) can be observed but are hunted and so suffer from a reduced lifespan. Dragons in more remote areas are poorly observed - although it has been speculated that dragons do not truly age, or at least enjoy a potential lifespan much longer than humans'.

Bonded Dragons & Dragonriders

According to myth, at the dawn of time the goddess of Love joined with the god of the Sun and created all life in the Known World. The goddess of the Moon , twin sister of the god of the Sun, was driven mad with jealousy and plotted to destroy all life and usher in darkness. To protect against her evil, the goddess of Love gave children to the humans who were empowered to bond with a dragon. The ability to bond with a dragon is a hereditary trait (although it is not found in every scion of the bloodline - it skips generations and is usually only found in a single child of a family if at all). There were always few dragon riders and they were imperfect and sometimes-selfish humans. Nevertheless, fiercely loyal to each other and sharing a deep communion, the dragons and their riders became a force for Love in the world.
Plotting over the centuries, the Moon goddess and her cults (aided by the unification of the Al-Djinn tribes) launched a pogrom against the bloodlines of the dragonriders. Searching for them using temple records and family myths, they assassinated, started wars and simply slaughtered people - eradicating the bloodlines from the world. For several generations, there have been no dragon riders and they are widely considered a myth.
Dragon riders were not any stronger or more powerful than a normal human (it is not known if any of the near-human species are capable of being dragon riders) and their dragons were normal dragons, but the bond they shared - allowing understanding and communication, as well as loyalty and obedience, was a great advantage. Dragon riders were highly respected as champions of good and Love.
Their dragons never grew beyond the normal size of a dragon - about the size of a horse - and the bond they shared meant that when one died the other did too. Although never explicitly confirmed, it was assumed dragons and humans were conceived on the same day - a rider would bond with a dragon when it hatched and he was in his sixth year. Dragon riders would feel an inexpressible and irresistible urge to travel to a dragon nest - even if the location were unknown to their conscious mind. In families where their bloodright was known, this would be prepared for - but in other cases children would simply run away and return with an infant dragon.