Image imported from derpibooru 
Original Description:
 
By this point in Pony history, the other three herds had formed permanent settlements, and were waging war on the very plains and grasslands that once fostered their Mother Herds. The Pegasi, in some attempt to torment and punish their earth-bound enemies, blocked out the sky with thick clouds, depriving the lands below of sunlight and warmth. Their plan soon got out of hand, and the Pegasi realized they could not clear these clouds, and even worse, were not able to fly past them to their homes in the sky. They, too, were trapped on the ground in what was soon becoming a true ice age. The war, which had lasted nearly a thousand years by now, slowed to a crawl. Ponies died in leagues from starvation and exposure on the tundras, and the Pegasi were blamed for the blight. To save their own tails from further punishment, the Pegasi blamed a false and mythic beast they refused to name for fear of summoning it, and the other Ponies ate up their lie in some hope that this beast could be defeated. In their fear of extinction, the races called an unsteady truce, at least until the long winter ended. Though much squabbling was had, and the shared bounty was small and unfilling, the joined races enjoyed a meager celebration of their truce. The Ponies celebrated for many nights, with a few romantic unions even forming between those of differing herds. Much to the surprise of the Ponies, after the merriment died down, mostly for lack of any more food to spare, the Pegasi found the strength to chase away the storm clouds. With full bellies and hope for the future, the Pegasi found a well of strength within them to push their falsely conceived weapon away, and let the sunshine in.
 
From that point, the three races decided to work together, under the guidance of six matriarchs: Puddinghead and Smart Cookie the Terrans, Clover and Platinum the Unicorns, and Hurricane and Pansy the Pegasi, who had led their own herds with expertise. These mares, known in the myths of Hearth’s Warming, worked together to build a united society, under the new moniker ‘Equestria’. The six mares built Equestria as a unified herd, with each race commingling and many hybrid foals born in the following seasons. The High Matriarchs, as they were called, went down in history and legend as spirits of bounty, change, and harmony, and are folk saints in their respective races. Some ponies even believe the High Matriarchs are all ancestors to the bearers of the Elements of Harmony, with some even being traced back through definable lineage, like Rarity who is truly related to Platinum through her father.
 
Following the events of the Thousand Year War and subsequent unifying of the herds, there was an extended stretch of peace amongst Equestria. All seemed well, and things were prosperous for more than five centuries.
 

 
The Matriarchs (top to bottom)
 
Hurricaine the Fearless: A seasoned and hearty Pegasus mare, Hurricaine was one of the Pegasus leaders who orchestrated the blocking of the sky, and is bitter and angry that her resulted in such a shameful failure.
 
Pansy the Gentlehearted: A young Pegasus mare that led her heard with a tender wing. Her dreams of peace and harmony among the three pony herds was one of the driving forces behind the success of the hearths warming truce.
 
Puddinghead of the Low Valley: Puddinghead is the oldest of the High Matriarchs, she did not really put any faith in the hearths warming truce, and only went along with the crowd as an easy way out of the pickle that ponies found themselves in.
 
Smart Cookie of the Western Hills: Smart Cookie was a young mare when she came to power, and has always led with logic rather than tradition. She was incredibly high strung and bossy, and her insistence on going along with the truce rather than following tradition may very well have saved all of pony-kind from death.
 
Clover that Grows on Sunny Hills: Clover, a young mare at the time of the hearths warming truce, was the leader of the lesser herd of Unicorns, and her cleverness and ability to work a crowd is credited as the most valuable asset of the High Matriarchs. She is remembered fondly by her followers.
 
Platinum Found in the Deepest Mines of the Tallest Mountain: Platinum was beloved by her herd, and her gentle manner was the real power behind Clover. Without Platinum nodding and repeating everything Clover said, the message of Clover would have never been received.
 
I wanted to also show off the way that the pony herds chose and formed their names, pegasus ponies choose a title that describes themselves when they come of age, terrans inherit a name based on the home range of their herd, and unicorns are named by their parents based on their appearance and their aspirations for them.