Which story would be the one that has the least anthropomorphism?

Good question. My first guess would be this one, but honestly, I am unsure. Mlp characters are already anthropomorphized, so it is uncommon for them to be more feral. This one is more alien culture, but does read somewhat xenofiction.
https://www.fimfiction.net/story/76290/celestia-sleeps-in
This one also has a decent amount of that. There are absolutely others, but I don't tract by that, so it will take me a while to find more examples.
https://www.fimfiction.net/story/35763/oh-to-be-old-again

7715789
Thanks, I read the first one, second one is in my read it later folder. Gotta change the priority of that one.

https://www.fimfiction.net/story/132779/onto-the-pony-planet
https://www.fimfiction.net/story/159365/pony-planet-side-stories
https://www.fimfiction.net/story/317740/silver-glows-journal - Probably the best one.
https://www.fimfiction.net/story/488706/saturday
https://www.fimfiction.net/story/460102/gamer-girl-shiroi
https://www.fimfiction.net/story/468234/practical-difficulties
https://www.fimfiction.net/story/448279/window-washers
Honestly, a ton of the stuff by Admiral Biscuit qualifies.
https://www.fimfiction.net/story/255419/the-substitute-demon - Warfare, Clop
https://www.fimfiction.net/story/224216/my-life-as-a-bipedal-quadruped
https://www.fimfiction.net/story/479815/everyday-life-with-guardsmares - RGRE
https://www.fimfiction.net/story/458572/my-farrier-lady-sunspot
https://www.fimfiction.net/story/501001/the-long-and-short-of-it - RGRE
https://www.fimfiction.net/story/391264/saving-equis - Clop
https://www.fimfiction.net/story/181310/my-secret-life-as-an-evil-insect-overlord - Clop
https://www.fimfiction.net/story/55377/blueblood-hero-of-equestria - Warfare
https://www.fimfiction.net/story/269344/four-hooves - More primitive and lack of civilization
https://www.fimfiction.net/story/480268/changing-expectations
https://www.fimfiction.net/story/302286/altered-visage - Clop
https://www.fimfiction.net/story/415985/off-the-mark
https://www.fimfiction.net/story/250542/straight-to-belle - Clop
https://www.fimfiction.net/story/492728/how-to-preen-your-princess
https://www.fimfiction.net/story/158287/whiteout
https://www.fimfiction.net/story/2857/mystaras-little-ponies-friendship-is-adventuring

https://www.fimfiction.net/group/197413/folder/4837/stories-of-horsey-ponies
This group is clearly what you are looking for, but it seems pretty small.

Honestly, a big issue is that I group more Feral behaviors and cultural differences together. So I have to remember the stories where ergonomics especially are different, moreso than culture, but some cultural differences are very much based on Ponies themselves.

And lastly, because I feel obliged to, This one. Not MLP related, but one of the best pieces of Xenofiction I've ever found.
https://archiveofourown.org/series/371546

7715824
Regular xenofiction is still fine, thanks. Some of those I am familiar with, the others go to their own special folder.
Yes, AO3 has some, I've been browsing there occasionally. Lots of centaurs and unicorns there. Bought a few books as well, whatever I could find on Goodreads.
The Guardian Herd, The Firebringer and Horses of the Dawn are good. Got a few non-equine xenofiction books on my wishlist.

7715841
Hmm. Could you link some of those AO3 Stories?

7715842
https://archiveofourown.org/works/22191634?view_full_work=true
https://www.archiveofourown.org/series/1020915

I think you will like this one, it's unique HiE
https://www.archiveofourown.org/series/1455115
Some Trixie fic:
https://archiveofourown.org/works/16673875

I can link more, but my AO3 "Library" is a buch of links, most of it I don't even remember. There are at least two good ones I will have to track down.

Pretty much the entire catalog by
https://www.fimfiction.net/user/29923/D+G+D+Davidson

All of his works put emphasis in world building ponies as equines, not humans who look like ponies.

Out of those this one:
https://www.fimfiction.net/story/104914/the-mixed-up-life-of-brad

Is probably the best in that regards, though its a HIE it's really Brad trapped in a truly alien world that keeps expanding. Unfortunately a he had a bad habit to not complete stories, but his are about the only ones i feel comfortable recommending even in that state because of how big an effort he put into making sure ponies act like ponies.

Of complete works, if you dont mind a few NSFW scenes, then
https://www.fimfiction.net/story/117975/an-apple-studded-diamond

The author is actually knows a lot about horses and developed the world in his stories to fit actual equine behavior in a less than industrialized society, it's an excellent read if you want xenofiction.

A more light read is
https://www.fimfiction.net/story/119354/dead-eye-darling

Rarity gets transported to the wild west, unlike nearly every fic on a pony on earth where ponies seem to be just talking dogs, she is actually just a pony, just intelligent and with a horn, with all that implies, exploring how terrifying it actually is to have the strongest and fastest intelligent being in a room being a horse instead of a human. It takes a weird turn at the end but its a fun read.

7715991
Thanks.

7715696
https://www.fimfiction.net/story/473866/mares-complaining-about-penises <- this kicked me to create this group
https://www.fimfiction.net/story/185272/the-chase <- ton of feral
https://www.fimfiction.net/story/12776/the-conversion-bureau-going-pony <- transformation to pony and experiencing changes
https://www.fimfiction.net/story/357269/romancing-the-pone <- one guy transformed to pony and cope with changes