Tribes of the north

Crystal ponies- similar to Earth Ponies, but their magic is tailored to the manipulation of all things crystal-related ie. crystal sheep, crystal berries, and crystal formation itself.

Steel Ponies- similar to Unicorns though hey sacrifice the ability to harness some of the finer aspects of magic, but are built more for the cold ie. longer feetlocks, thicker coats, canines (for eating meat). Are easily distinguishable buy their more monotone color palates, and smooth curved horns. (Sombra was one, the last known one)

Imperials- similar to peggesi but they lack the ability to munipulate the weather, but are built more for the cold ie. denser coats, "fluffy" ears, larger wings with denser plumage, "fish scale" hooves (for climbing glaciers), canines (for eating meat).

Northern tribes origins:
When the three tribes united there were members of the Imperial Peggesi Legion that did not agree with the newly formed Equestria seeing it as a dishonorable defeat, not liking 4 to 1 odds or the prospect of fighting their brothers they chose to head to the northern tundras, soon after they adapted to a nomadic life style (similar to the Inuet) coupled with their accelerate natural evolution (due to the norths wild magics) and within a century the Imperial's were a self sustaining isolated tribe preferring to keep to themselves and avoid Equestrians, stating that their fight has long sense passed. However with return of Crystal Empire (their only previous contact in the north) some tribes have taken it as a sign and started to integrate into both the Empire and Equestia.
A similar story for the Earth ponies becoming the Crystal ponies and founding the Crystal Empire, and the Unicorns becoming the Steel ponies and founding several small fortified city states.