During the final season of My Little Pony I tried to create a military-centred sub-setting for it. Not truly AU because it takes place in times that is almost obscure in canon while one of inspiration sources - Equestria Divided - simply takes canon characters and places them at various post in fighting houses. And while the rosters themselves wait for compilation and translation from russian, I managed to make this as some sort of teaser about how actually Equestria was made.
 
So, the first thing you need to know is that I think that inintially the three tribes (unicorn, pegasi and earth ponies) lived in the “Old World”, the eastern shore which is shown on the Cutie Map while Equestria is like America. Specifically, the earh ponies - at the north-west part of the continent (obstructed by chair on the picture) as the location of Rockhoof’s village also suggests. And they were supposed to be germans because they had a chancellor in the “Hearth’s Warming Eve” which also makes sense as in that village ponies with Mjolnir, Ygdrassil and valknut (Rockhoof himself) were shown. But enough about them, let’s go north. There lies the Serpent Sea which has been named so not without reason. It has a lot of islands, which, according to a legend, are the spine of truly monstrous serpent who barely fitted in the ocean and that’s why more of his kind like to settle here. They were drakes, wyverns, wyrms, salamanders and whatever else vastlanders called them later, while settlers were more interested in how to get rid of them. All of theese settlers were earth ponies because unicorns were simply not physically strong enough to survive constand battles and and harsh weather and also had been utterly terrified by alien nature of local fauna (similar to these poor things in the cages who were not released here seememgly because had no magis to drain). As for pegasi, their homeland was simply too far south and tiny pieces of rocks not worth the effort of founding weather control colonies which would fight wild blizzards for most part of the year (like the one that assaulted Crystal Empire during season 7 start).
 
So colonization of Serpent Sea went on for centuries, teaching people stubborness and literally forging friendship when fire because when all of your property could be easily destroyed you really learn to value those who help you to restore it. But while actual three nations were risen as a united kingdoms, nhorse society remained tribal due to isolation: any naval expedition they needed to set for diplomacy attempt could end up not only drowned or frozen to death, but also simply eaten. They shown on the continent only occasionally to pillage or trade for food. Mostly the former because vastlanders were disgusted by nhorse omnivorous diet and all aforementioned was also the reason why the earth tribe did not try to annex them.
 
But the farther North, the more stubborness turned into insanity. Those who managed to consolidate on the norhen shores, in land that was called Hel, suddenly found here compeltely wild and primordial griffons instead of usual threats. They tried to colonize islands before ponies, driven from their homeland by far more organized pegasi. In Hel sometning was wrong about nature, but not in the way unicorns thought about the islands. It was actually hate and warlike spirit of generations that condensed there in form of unnatural cold since the time no chronicles write about. As nhorses proved themselves as strong warriors and refused to leave against all odds, at one point they were offered a deal - unholy strength for eternal service to Ice Queen. So they started to raid south not only for supplies but for the violence itself, trying to earn grace of their godess and believing firmly in natural selection. This became serious threat soon after their southern brethen managed to solve their main problem in “can’t stop - lead” way and so started to confederate, finally able to travel fast and live rather safe. And this is the situation at the time of the Hearth’s Warming legend.
 
P. S. Yep, you can call it a crossover if you want, and it can be rather weird and stupid, but I can do nothing with my obsession.