Iron Dragon

The Midaevian Empire is, at its core, an exercise in order through control and unity through strength. The Empire depends on its ability to project power, and nothing in the world represents power more clearly than a dragon. The draconic race has existed since long before the elves appeared on Altharad or the first humans set foot on Erru. The oldest tales speak of ancient battles between fell titans and gods, bolstered by their dragons allies. The Gods and Titans may have long since vanished, but the dragons are still a common sight in the wild places of the world.
It was common practice in early Midaevia for champions to venture into the nearby Blackthorn Mountains and seek out a dragon to slay, thereby proving their devotion to the Elder One. Dragons were considered the natural enemy of the Empire; powerful, sentient beings with a lust for domination to rival that of the Midaevians. For decades, the Empire viewed them as nothing more than a menace in need of extermination, but as more and more dragon corpses were hauled back to Verus where their scales, blood and organs could be harvested, a small faction within the Ministry of Enlightenment began to consider if they might serve another purpose, and started secret experiments below the streets of Verus in pursuit of this question.
Eventually, their activities were discovered, but by then they had something to show for their efforts. Through dark magic and immoral science, they had successfully created an entirely new species of dragon; fire-breathing, with iron-coloured scales and bright blue eyes, similar in appearance to bronze dragons. However, unlike bronze dragons, or indeed any of their metallic or chromatic cousins, though they could understand languages and speak them to a degree, the iron dragons possessed only rudimentary sentience, a far cry from the staggering intellects of the silvers, reds or golds. Their needs and desires were simple, and so breaking them in for use as war-mounts also proved simple.
Physiology and Society
The size and weight of irons, like all dragons, varies as they age. Wyrmlings typically weigh around 60 kg, the same as an average human, and are equivalent to large wolves in size. Other dragons reach maturity at 100 years of age, at which point adults weigh about 19 tonnes and are the same size as a two storey inn. However, in order to preempt the needs of an Empire in constant need of reinforcements, when an iron paired with a knight hatches, the Praepostors  charged with overseeing their development alongside the knights will introduce "adjustments" to their physiology. As a consequences of this, a dragon's development to maturity accelerates by a significant factor. This has obvious repercussions on the dragon's psychological development, but the Empire sees tempering this as the rider's responsibility. And should a knight fail to properly raise the growth accelerated juvenile causing"accidents" to occur, well...it is easy enough to put down the creature after it has finished eating its rider and find new candidates. After reaching maturity, whether naturally or synthetically, the dragons will grow slowly but gradually as they age. So called "Ancients", those who have seen 700 years or more, can reach weights of 50 tonnes and rival mountains in size. An endless succession of wars has ensured that only a scant handful of irons have lived long enough to reach "Ancient" status, such as the dragon of the Grandmaster of the Iron Knights; Steelwing.

Left to their own devices, they tended to form small units based around a mating pair and their offspring, who they would raise while they were wyrmlings but cast from the nest once they were old enough to hunt for themselves. Each family keeps to their own hunting territory, rarely venturing beyond their borders unless to meet the challenge of another iron dragon.
The Emperor's Dragons
Left to their own devices, and with no natural enemies for a thousand miles in any direction, it is likely the iron dragons would eventually breed out of control. The demands of the Empire serve as a counterweight to the growth of their population. The Ministry of Contention keeps scrupulous records about every breeding pair in existence, and takes two out of every five eggs from every clutch, leaving just enough to maintain a sustainable population without allowing their numbers to grow too large. Those who are taken as eggs and raised in the ranks of the Order of the Iron Knights as beasts of battle, training to fly through hails of ballista bolts and magical storms in pursuit of the enemy. It is here that the peculiar nature of the iron dragon shows itself; for all the savagery they display in their natural habitat, they seem positively civil when travelling through Imperial territory, hunting only when their rider permits them to do so, obeying every command no matter how frivolous.
This behaviour has led the other dragons of the world to dismiss the irons as unnatural copies of true dragons. But while their submissiveness may prevent them from achieving the same levels of power as their independent-minded cousins, only a fool would dismiss the threat possessed by a fully grown iron dragon, especially when encased in dwarven-forged armour and accompanied by their magically-enhanced rider.