Those blessed with access to the restricted section of the Canterlot Archives, or with a reader's omniscient perspective, will know that the list of Equus's sophont species Twilight gave to Cordelia was somewhat incomplete, for all that it is the list of species which any well-educated Equestrian savant would be able to rattle off. Perhaps the most prominent omission, other than the strange species of the uttermost West, would be the yaks of Yakyakistan.
Another omission is the caribou. (Or rather, was the caribou.) Yes, those caribou, who had developed into every bit as disgusting and terrible a culture as that series portrays them. They just made three fatal mistakes, unless one counts their fundamental error (or fundamental misfortune) of existing in the same universe as Luna Versuta, Avenger of the Herd.
In the past, the caribou dwelled in the lands north of the Crystal Mountains, and as slaver cultures tend to do, they preyed on their neighbors - those being the dragons of the Withered Wastes (which extended less far south back then), Yakyakistan to their west, and the Crystal Empire and the future Stalliongrad to the south. They were notably unsuccessful in attacking the major cities (the Crystal Heart repelling such loveless creatures very effectively, and proto-Stalliongrad having thick walls and cannon), but their hinterlands suffered greatly from raiding.
Their first mistake was in assuming that the alicorn Princesses concerned themselves only with the ponies of Equestria, rather than all ponies everywhere. Not that they would have taken them seriously anyway, being mares ruling, but that's less a mistake than a persistent state of stupid.
They did, however, send a diplomatic mission south to Equestria. To their eyes, after all, it looked like a soft country ripe for conquest, without any real power. It should perhaps be noted at this point that "diplomatic mission" in this and various previous cases meant "some caribou able to control themselves and their mouths well enough to spy".
In the modern day, it is generally accepted both that diplomatic immunity for diplomats accredited to the Day and Night Courts includes a solemn promise that Princess Luna will not walk in your dreams uninvited, with the awareness that if your dreams are particularly charged, there's no guarantee that she won't overhear things from the dreamscape, and as such, diplomats are generally chosen from those possessed of great serenity and ability to look at pretty much anything with a calm mind.
The former rule exists because of the caribou's second mistake: sleeping in Canterlot, where their dreams of what exactly they would do to their new anticipated conquest - and the Crystal Empire too, for that matter, once they had the ability to hit it from both sides - were completely open to the Princess of the Night, resulting in their rapid ejection from Equestria with a stern warning never to return.
And their third mistake? That would be believing they were safe after having returned to their homeland, and to go on plotting havoc and war. After all, they knew Princess Celestia's reputation; and through the eyes of their comically macho culture, kindness, gentleness, and second chances look almost exactly like weakness. And if one weak mare cannot possibly be a threat, what threat could her younger sister be? Ridiculous!
(They may also have had some trouble with the concept of diarchy, and assumed that only the elder could actually be in charge, and could therefore keep the younger in check. Oops.)
Three of them found out.
The rest of them never understood exactly what happened, because Luna's epithet isn't Versuta for no reason. The only reason to get Equestria's armies entangled in a long war was to preserve lives, and she was in no mood for that. From the heights of the Crystal Mountains, the Cunning Moon whispered ambition, delusion, and paranoia into the dreams of the caribou, stoking dissent, jealousy, fear, greed, and so forth, and raising tensions until one final spark eventually exploded into violence, and in one bloody night of fighting the caribou tore each other apart. Only the aforementioned three nobles survived in long-before prepared hidden bunkers, forcing Luna to slay them personally. She still prefers not to discuss the matter, finding it quite embarrassing.
(Those of their slaves still functional in mind, prepared to hide from the eruption by dreams of their own, scavenged all the necessities from the ruins and began the long trek south into the Empire, shadowed and secretly helped by Luna's agents.)
As for how this affected Equestrian relations with Yakyakistan and left them in a rather sorry state for many years thereafter, resulting in they, too, being omitted from common knowledge… well, that's another story.
It is, however, entirely untrue that Luna still keeps the caribou king's skull, dipped in molten silver, in her liquor cabinet.
(It's on her desk.)