I spent all day composing this in the comments of the new chapter, and figured I may as well get everyone else's take.

SFaccountant deserves credit for incorporating the yaks. It's actually really handy how Yakyakistan can slide in or out of any story's ongoing canon. The yaks are violent traditionalists who live in the frozen armpit of the world, so it makes total sense why nobody would ever mention or run into them. But now we're looking at Centaur III on a global level, starting with regions closest to Equestria, so this is the perfect time to mention the yaks. The yaks are cranky, beasts of burdens that can take large amounts of weight, like to fight, have huge bone protrusions, are super hairy, and probably smell absolutely zogging rank. Just like the boars the orks love to ride, even at the point where they have space ships.

The orks should try riding the yaks as mounts. This would not be in anyway a friendship, as the sentient yaks would still probably be treated as beasts of burden (and war), which would probably be constituted as slavery. But that's still pretty much the relation that Luna and the Mane 6 (and perhaps all of Equestria) now have with the 38th. The yaks and orks would have the same thing going on, but without the advances in management and technology, the trappings of polite society, and the extra steps of oaths and contracts.

It's not that orks actually let take prisoners out of curiosity if they can utilize them, like the 38th do. I'm also not saying this is a solution that the yaks would really think of, or even be fond of. Just hear this out: the yaks are overly courageous, prideful and violent (it's why Party Pooped wasn't my favorite, I didn't buy why they had to be 'friends' with the yaks). There's no way their warriors would go for a running-battle, guerilla-style campaign if they could help it at all. MAYBE they would stick to harassing the Iron Warriors, who are clearly better armed, armored, and actually conquered the world. But they probably wouldn't have much fear of the orks, who have more scattered numbers and were recently trounced. So when the orks find their way their way to the frozen north (maybe or maybe not encountering the Crystal Empire), the yaks decide they won't have any of these intergalactic pests. They charge loud and proud into the meandering crowd of orks. But where most armies would scatter when rammed into unexpectedly, this would just kick the orks out of their malaise from their recent crushing defeat. They look at the charging, screaming melee, and recognize "OOO! 'Dis iz our jam!"

They holster their guns, and run straight at the oncoming rows of horns. The yaks gore the ork bodies, and stamp and crush the fallen. The orks throw and kick off whatever lands on top of them, and jump onto the backs of frenzied yaks to punch them in the heads. The yaks have a hard time actually killing off the orcs, and the orcs aren't really using every advantage they have, sticking with blunt force for their own pleasure. There's a body count, or course, but it's rising slowly, not helped by the snowy conditions. The monotonous melee devolves into one-on-one face offs, with the enraged yaks falling on their most basic instincts of charging, then coming back around, and the orks deciding this makes for a good contest to see who has the thicker skull. The rest of yaks are knocked out, surprisingly not dead after a protracted fight with orks. The greenskins take the concussions like happy drunks, and when all the bovines are laid out unconscious, the orks haul them off with big sloppy grins. Then we switch POV to hours later, where one yak is coming to. He feels a tugging on his face, and a weight on top. It's an ork with reins, who then starts kicking him in the sides, shouting "Go hairy-horn-squig! Go!"

Then the yaks just give up and go with the flow, because there's no way out of this. The orks aren't even afraid of the yaks. They would only get bored and EAT them after too many fruitless attempts of getting them to follow orders. To me the yaks are violent jerks, but still a hilarious to watch when we're not being told we have to be friends with them. So I see them getting on with an ork tribe relatively well.

Oh, and it's worth noting that yak racing is a real thing.

Holy crud, when I composed that encounter, I was just imagining it based on reading about orks in novels and fanfiction. But apparently headbutting really is an essential part of traditional orkish animal husbandry!

http://warhammerfantasy.wikia.com/wiki/Savage_Orc_Boar_Boyz

Savage Orc Boar Boys
"Instead of traditionally breaking a Boar as a mount before combat, the Savage Orcs prefer instead to stalk and ambush a wild boar and headbutt the beast into submission."

I've been going through WH Fantasy lore on account of the new Total War game. I was so happy to run into this!

Then these guys show up.