Before we begin, I'd like it to be known there are NO real-life racial connotations intended here. I based these guys off the Kingdom of Arnor and the Dunedain from Middle-Earth. I've come down with a bit of a Tolkien craze since playing Shadow of War.
Just so we're clear.

It’s a fact. Most ponies don’t go around with dark coats. So rare is such a pigment that it’s become synonymous with the unfamiliar, the suspicious...
Even, on more that several occasions, the outright dreaded.
Pony coat colours are dependent on a variety of factors. Thanks to the millennia of evolution of ponies and their society, a pony’s richly diverse genes can cause his or her coat and mane to come out any colour that any ancestor of theirs once possessed, no matter how old. The likelihood is slim but it is indeed possible.
Despite this, however, there are still very few ponies of dark coats and even among the urban areas, they aren’t called dark. A lustrous shine and a streamlined build make the dubious connotations impossible so one gives them different names. ‘Silver’, ‘Ebony’, ‘Jet’, ‘Slate’ and ‘Onyx’, derived from figures of the Ancient Crystal Empire.
Indeed, this has given rise to the belief that the Empire is where any good-natured dark-coated pony must originate from.
But this is not so.

An often forgotten but pivotal point in Equestrian history, before the Unity of Tribes and the Rejuvenation, was the Rise, the Fall and the Eternal Struggle of the Dun Herd.
The Earth Ponies battled nearly every harshness and vileness the ground they trod could throw at a pony in their journey to find a home to call their own.
Of the three tribes, it is they that still possess the most insights into their history before the birth of Equestria quite possibly because most of it was buried. And what is buried can be dug up.

The Earth Pony origins began in the west, their leaders believed to be descended from the nobles of Hyraxis, an ancient city sunken beneath the waves and tides countless millennia ago.
Their first attempt to build a kingdom where they landed, on the Stormy Shore, ended quickly. If it was even possible for a kingdom to be built on an eternally tempestuous spit of land that grew no crop or hid no metal, it wouldn’t be worth the effort. They ventured inland through the forests and into the vast grasslands. There they carved out many separate states, one of which becoming the Grand Republic, who would one day, under Chancellor Puddinghead and Smart Cookie, unite alongside the Pegasus Junta and the Unicorn Kingdom.
But the Earth Ponies saw no point in giving up the lands they already had. Drawing up trade routes and supply lines between each of their city-states, they charted up their territories between their triumvirate in Canterlot to the westernmost point in Equestria, facing the great forests south of the Smoky Mountains.
Dunholm.

Dunholm had started out as a simple settlement from which to spread out from the harsh and mysterious far-west to venture into better pastures. But their close proximity to lawless border sites, monster-infested forests and haunted ruins made it a haven for adventurers or adventurers-in-training, hungry for glory and excitement. The city was bolstered and defended and became one of Equestria’s strongest settlements.
Behind its high walls and heavy gatehouses, Dunholm was an imposing and sprawling metropolis characterised by dark and bizarre architecture with the tops of its towers and sepulchres covered by mist. Visitors would often claim the city had a ‘grim’ atmosphere to it that put off visitors with little interest in danger and death-defiance. The city was overseen by a Castellan who was guarded by the Sentinel of Dunholm, the most dedicated officer and war leader who oversaw the armies and defences of the city while those venturing out of the city into the lands without law and order would find work at the Hunter’s Guild were they would receive clients, missions and payments.
The Grand Republic prospered through Dunholm’s monopoly of adventurers and it’s believed some members of the government sought to ‘get one over’ on the Pegasi and Unicorns through such means.
Until the Invasion of Tirek muddled every plan every figure in Equestria had for its neighbours.

Dunholm’s expertise in military was known to Tirek and one of the first courses of action he took was cutting the city off from the head of the Republic. The assault was commanded by Grogar the Treacherous who took hold of the ruins surrounding the city and awakened the dead within. Attacking the city with an army of those who had perished in the adventures the city had tempted them with, Dunholm was set into a period of constant watch and battle, unable to break the Tirek’s frontier and aid Equestria. Afterward, he forged a pact with a savage long-haired unicorn tribe over the Republic borders, the Adharcan, hated enemies of the denizens of Dunholm. Through his machinations, Dunholm was left virtually alone, forced to fend for themselves.

Yet this was nothing they did quite so well. Working through guerrilla tactics, they let invaders into the city and destroyed them almost instantly as they did battle with Grogar, fighting deception with deception.
Throughout this, they were commanded by a Castellan named Lady Sable, an expert tactician who was prepared to do anything to protect her subjects, and a Sentinel named Dusken Blade, often simply known as Dusk.
Their peregrinations infuriated the Horde and eventually Tirek was called to take to Dunholm personally. But there, Sable and Dusk prepared their ultimate endgame weapon.

A great alchemistical superweapon that enveloped the city in a great red smog, gripping everybeast in a foul, bloodthirsty frenzy. Ally turned on ally. Brother turned on brother. Tirek staggered back as his own kinbeasts and honour guard drew blades on him as he did the same for them, cutting down several of his foremost lieutenants. Only Grogar saw through it and determined the city had been abandoned.
Despite the humiliation, Tirek still counted this as a victory. The Republic had lost its station of most able warriors. But through the efforts of Dunholm, the Earth Ponies, Pegasi, Unicorns and their Princess, Laurelore, had been given sufficient time to bolster their strength and were now ready to take the fight to the Horde leading to a war that would lead to his first great defeat and capture.

Unsatisfied with the way things had turned out, Grogar ensured Dunholm could not and would not recover. He gave a task to one of his most loyal followers, a mule, so it’s said, whose name has been forgotten by the ancient texts. This mule ventured into the dells before the mountains and began his work. For his service, he’d been gifted with one of his master’s Cursed Colliers, infused with his dark power, and began drawing any locals to his service. Finding a petty kingdom with an ailing monarch who required an advisor, he slowly worked his way to the top and founded the Kingdom of Arvator, an ancient equine term for ‘House of Eternal Slaughter’.
The mule became known as the Pale King and saw to it that the suffering of the ponies of Dunholm would be without end.
So it was that Sable and Dusk ruled over a herd without a home, a nomadic society of rangers, gatherers, druids and their families, a civilisation set in a wild and untamed fashion to this day and a race of ponies known for their distinct dark coats.

There are many variations, as there are in mainland Equestria, but one would not find any coats of blue, pink, yellow or multi-chromatic. One would find in abundance coats of black, bay, chestnut, cream, grey, mahogany, blood-red and patterns of dappled, brindled, speckled, flushed and patchy fur.

Numerous times, scholars have debated on the line between ‘horse’ and ‘pony’ and wondered if the Dun ever crossed it through their regression but their studies are inconclusive. Regardless, the Dun have served as the bulwark of the West since the Founding and there are few who know of ponies who’ve survived through hardships such as theirs.

Sable and Dusk were married soon after the Flight from Dunholm and birthed a daughter, Glade, and a son, Midnight.
Glade and Midnight were trained in the ways of the warrior from a young age. Glade was a self-confessed tomcolt, interested mostly in front-line fighting and monster-slaying. She won her first battle against Arvator at the age of twelve, slaying a prince of the Adharcan and the drake he rode. Midnight, meanwhile, had a better mind for the scholarly arts, fascinated in alchemy, scrying, smithing, navigation and other studies. It is this that caught the eye of Star-Swirl the Bearded, venturing to the Dun Valley on his travels. Midnight was tested and taken as his student at the age of thirteen and was sent to the Royal Academy at Canterlot. The rest is history. Midnight Blade learned under Star-Swirl as well as his uncle, Vorpal Blade, the Dun’s Ambassador to Equestria and a renowned duellist and philosopher, and rose to become Guardian of Princess Luna and Lord Commander of the Night Guard.

The Dun crowned Glade the first Queen of Dunvale and for the majority of her reign, she ruled well and brought glory to the Dun and death to their enemies. Her daughter, Dell, did the same, opening trade and alliances with the forest deer. The reign of her son, Droy, was the same until a succession crisis broke out after his death. Dunvale was broken into three separate kingdoms.
Dundale, led by Princess Fellwyn, claimed the south, the valley before the trees and its roosts.
Sorell, led by Prince Chaster, claimed the east, market-towns and fields.
Vair, led by Princess Saedia, claimed the north, fens, barrows and rivers.

As the land was wracked with dispute, the return of the Pale King seemed a welcome sight as it would surely spurn the squabbling siblings into unity once more.
Yet, as the Arvator host swarmed Dundale, this belief was crushed. Fellwyn was slain by the Pale King, reinforcements having failed to arrive even in her last moments, and her daughter, Falene, was captured.
Only after this, Sorell made their move, Chaster only seeing fit to claim the centre once it was open for him.
Yet his term as King of the Fields and Dale lasted only a month as the worst was yet to come.
Saedia had offered herself to the Pale King as an ally, claiming his champion, Mordantine, as her consort and lending her troops to taking Dundale, killing her brother in the process, while the Adharcan crossed the borders and claimed Sorell under their Warchief, Dargungar.

The Dun had fallen upon themselves and their kingdom was lost. Hope seemed distant, if in reach at all.
But it was then that a hero returning from the war, having cast aside his new life and returned to the old, answered the call.
Midnight Blade came home with a vengeance, as an undying batpony, by what means his friends and enemies knew not of.
Yet his falchion, the blade his father had made famous, was still swift, keen and deadly.

So began the Dunvale War. A war that brought destruction to both kingdoms yet the Dun lived on and were able to recover.
Saedia, Mordantine, Dargungar and every lord, champion and monster pledged to Arvator were killed and the Pale King banished from this world, his return doubtful and dubious.

Until the return of Princess Luna, Midnight Blade spent a great deal of his time among the Dun as an eternal guardian of his kind, watching over them until the call to return to Canterlot and his Princess.
Falene was rescued and returned but lived on as Princess of the Dun only for half-a-month, crippled and frail after her capture. Her daughter, Farfalla, assumed the throne and the line continues up until the reign of Prince Flaxenhart during the First Raptor War whereupon the history is unrecorded and Mr Midnight Blade does not wish to speak of whatever transpired.

There concludes the story of the Dun ponies.
Whether they are gone from this world or living among us now they are far from as dreaded as the tales say.
A Canterlot Chronicler (Believed to have been Pompencirque M. Stance but this is unconfirmed) once said of the Dun...
‘They are as fierce as the stormy winds that blew them onto our shores, as cold as the moors as they now dwell upon and, let there be no uncertainty, crueller than any invader Equestria has seen.'
A speculation of an ignorant time or else an outright lie.
[Note: I, personally, have only my interactions with perhaps the only full-blooded Dun pony living in central Equestria; Lord Commander Midnight Blade, to go on. He can be fierce, certainly, and in some ways cold. But cruel? Never.]
But one such speculation is very much accurate. Their kind have endured much and suffered more and this has given rise to the term...
‘One cannot stain a dark coat’

Interesting. And nice reference!

6176001
I felt like it would be interesting to provide depth to ponies with 'realistic' coat-colours, almost like ancient or tribal ponies in a way.
It seems that dark-coloured ponies are probably the second-smallest chromatic group in Equestria. The first being green.
I've given green ponies a prominent history in Stirrope. Guys like the House of Armbrust are descended from Western and Central Stirrope's tribes and the earliest kings of the region (Essentially a Goth/Frank/Teuton equivalent. I'd better have them bring down a Late Roman Empire equivalent, a la' Battle of Teutoberg Forest)

6176028
Smart idea!

6176031
Yeah, and I thought, since the Apple family have a few green ponies, you'll notice Grandpa Apple wears German/Swiss garb. Though he's not green, it could point to certain origins.

6176034
Good point.