[This post uses some names and such from 4e D&D and the Hobbit. So if you wanna see where I was inspired somewhat, check out the 4e PHB and such.]
In my world, Lizardfolk and Dragonborn are the same race. They are both cold-blooded and immune to most poisons.
But if you ask one of them whether the two are the same, they will tell you otherwise.
Originally, there were only Lizardfolk, which organized themselves into various swamp-based and island-based tribes. They were a shamanic race with a rich culture and diversity.
Then, a prophet among them, named Arkhosia, began preaching a history and origin of the Lizardfolk. He bore a pair of great wings on his back, and said his people were born of dragons, not lizards. They were made as disciples to dragons, and crave their warmth (that is why they are cold-blooded). The dragons are a part of them, and it was their commission to take these dragons under their wing, so to speak, and focus their power for the building of a great civilization.
For you see, dragons are a terrible kind, prone to greed and sickness when exposed to wealth. Their power is unmatched, and if any race could befriend these serpents, then surely they would rule land, air, and sea.
The prophet Arkhosia said he was given instructions in a vision. Instructions for how to treat this dragon sickness and rehabilitate the monsters into loyal allies of unmatched strength. Arkhosia's words spread throughout the Lizardfolk tribes, and soon thousands called themselves Dragonborn.
Many dragons were pulled from their lairs and forced through a sort of withdrawal. But once through the ordeal, the serpents showed a bright intelligence and trustworthiness.
Upon the backs of dragons, Arkhosia built an empire. Rarely was there violence under Arkhosia's reign. He was a master of diplomacy, and sought only to unite dragons and all other races. He spread riches equally throughout his empire, so that the dragons were not stricken by exposure to hordes of wealth. He built no capital, and wandered the empire most of his days, protecting the people from monstrous threats.
Eventually, after a few hundred years, Arkhosia was seen no more. Some say he died alone but for his dragon, content. Others say he now roams the skies, born aloft for years at a time, and rarely seen. Others say he made his way to the underworld, in pursuit of a sorcerer of unspeakable power, to kill him, or perhaps befriend him? The stories all meld together.
In Arkhosia's absence, the empire remained strong, to a point. Soon, though, Dragonborn of ugly aspirations sought to expand the empire by force, and rule their territories more stringently than did Arkhosia. These Dragon Lords met at a single council to speak of their plans and the future of the empire. They came to only one agreement: that the empire should be named Arkhosia in memory of its founder. From there on, the discussions made no headway. Instead, they flew back to their respective corners of the world and ruled their lands as feudal lords, collecting wealth and exacting terrible violence against all who opposed them.
The remaining leaders of the world made an alliance to fight back the ever expanding and extremely violent Dragon Empire. The means by which they fought back ranged from valiant combat to devilish magic, and in the end, they merely survived: the Empire of Arkhosia fell on its own sword. When the dragons could no longer resist the temptation of riches, and the sickness ripped their psyches into monstrous minds, they turned on their own, burning the empire from within. The dragons destroyed everything but the gold and valuables, and stole away with their take into mountains and forests and deserts and caves.
Arkhosia was no more.
If you see a scaly humanoid walking among you, it is surely a Lizardfolk, a descendant of the tribes that never converted. They live in very much the same way as they did before the empire ever come into being.
But there is a spark, twisted by its own undoing, that still burns in the darkness. A deranged few Dragonborn still roam the world, cultists of the vilest orientation. They seek out dragons and dominate them, convinced that this was what the prophet had truly meant for their kind. They transform the dragons into horrible beasts through dark rituals and let them loose on the land.
They trek from ruin to ruin of their lost empire, practicing their rights in secret. Arkhosia is not gone, they whisper, and he shall only return once the land is scorched to ash.