Personally I see these creatures as the MLP version of Dwarves from LOTR (The traditional Unicorns make me think of Elves).
Chapter 4: Centicore (Yale)
Living within the mountains of Equestria themselves the ancient Yale form tribes that work and dwell within the rocks. Stubborn, secretive and wary of outsiders stealing their precious metals it is said that the species are related to the Unicorns of old however why they became subterranean is not known. It is assumed to be due to their association with earth based "magic" of lay lines.
Part of their earthen magic appears to be immunisation from a Cockratice's petrifying gaze, they will remain rooted to the spot however the stone casing will not form over their bodies. If a Cockratice finds a Yale asleep however and chooses to sting him/her the Yale will go completely blind when it awakes. The eyes of the stung Yale will become a pair of white marble orbs, the Yale will be unable to see any longer.
Instead of possessing conventional magic like their single horned cousins the Yale's magic lies within it's pair of horns, allows it's horns to move, swivel about it'd head. A horn may be positioned and used as a lance while the other rests upon it's back. Damaging a Yale's horn causes that horn to lose its magic and remain stuck in its last position (The Yale will not amputate the dead horn, the horn is left as a symbol of shame for allowing damage to come to it- NV).
Stocky in build making them just shorter than an average stallion, covered in shaggy fur Yale are considerably buffer and bulkier than standard ponies, even female Yale retain this bulk. They possess shaggy thick manes and lion like tails the colour of the mane and tail match just as the colour of the horns, cloven hooves match. Yale usually come in earthy colours such as browns, greys, maroons, shades of auburn, occasionally golds. Beards and tusks are often seen in both genders. The eyes of Yale are squared with a slit pupil, the colour of the eyes often seems to match the gem, metal or mineral that that particular Yale has a talent for finding and working with.
Yale tribes are commonly composed of blacksmiths, metalworkers, jewel smiths, goldsmiths and miners. The gems, minerals they mine are crafted into fine jewelry, trinkets, embossed into armour, weapons which will then be taken down into the cities, towns to sell, trade. (The Yale themselves do not usually go down to the towns, they often have a pony liaison who does the business for them. Whether this is due to discrimination due to their imposing look, or just because they really don’t like outsiders is hard to say- NV)
Due to their occupation one would expect conflict with the also subterranean diamond dogs however the two groups are peaceful, will enlist in each other’s help at times (for a price of course) they will happily do trades with the diamond dogs since the DG can get deep enough to unearth diamonds which fetch the highest price in jewelry, the diamond dogs lack the skill to craft such items (The relationship between the two is an almost symbiotic business one, DG cannot craft jewelry to save their lives, Yale cannot dig that far down due to the gasses- NV).
They will decorate their own manes, bodies, tails, horns and beards with things they have crafted from the gem or mineral they are most associated with akin to a Pony cutie mark. Although they do not have family names the last name of a Yale is the gem, metal or mineral they are most associated with i.e. Topaz, Gold, Silver, Sapphire, Ruby and their job i.e. Cutter, Sniffer, Miner.
Afterword from Nothing Ventured:
"An unusual race, not particularly aggressive but distrusting of outsiders.... unless you can offer them something in return they may just send you back out of their mines. I heard rumours they often have dragon problems... particularly in the areas that are rich in gems, precious stones"
Prologue: the Author
Chapter 1: Wendigo
Chapter 2: Qilin/Kirin
Chapter 3: Manticore
Chapter 4: Centicore (Yale)
Chapter 5: Peryton