The four distinct races, which were now undeniably Pony, were the Pegasus, the Unicorn, the Zebra, and the Terran. Each race lived for centuries believing their breed was the only one to survive the Great Upheaval, and deeply mourned the loss of innumerable sister herds. It came as quite a shock when the different races began to converge approximately five thousand years ago when the sea levels began to dip some, allowing safe and easy passage across the lands for any race, not just the Pegasi. Unfortunately, each race perceived the other to be tainted by some malignant and evil magic. Pegasi were believed to be terrors of the sky, who crossbred with the predatory griffons for power. The Unicorns were thought to be hosts to parasitic crystal creatures meant to serve the dragons. Terrans were believed to be cowardly and ungifted, choosing only to leave the valleys when there was promise of food and pleasure in the wider lands (Terrans were notoriously plump and thick-coated compared to their sister races). The Zebras were thought to be unsociable and harsh, with their nomadic lifestyle and deep connection to the earth believed to reflect unintelligence. Soon these unkind rumors lead to skirmishes, and then battles, and soon, a full on war. Somehow, perhaps due to the nomadic lifestyle of the Zebras, they were for the most part excluded from the squabbling of their sister herds, and were soon forgotten entirely.
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The Unicorn: The crystal core of their horn acts as a magic conductor, allowing for precise spellwork. The skull of a unicorn has an opening where the core connects with the frontal lobe of the brain, and a thick keratin sheath over the core. If the sheath is broken and the core exposed, the core can become infected and need to be removed, exposing the brain. Modern medicine uses broken pieces of the horn to cover the brain and patch the skull. A broken horn can cause chronic migraines, and an improperly removed core can cause uncontrolled magic outbursts as well as severe nerve pain and in some cases, permanent nerve and brain damage. Lesser horn problems can be found in unicorns whose core is undersized or has a poor connection to the brain. These problems can be corrected with therapies and enchantments.
The Pegasus: The secondary flight feathers of a pegasus wing act as a conductor for magic, and is directly rooted in the skin and bone of the wings. These feathers, which are iridescent in color, can allow a pegasus to walk on clouds, do weather magic, and affect the mass of the body to allow for easy and efficient powered flight. Pegasus ponies born without, or insufficient flight-feathers cannot fly, but can often walk on clouds and sometimes do basic weather magic. There are two types of feather disorders, one where the flight feathers do not come in at all, and one where they are severely stunted and do not work outside of cloudwalking. Some pegasus ponies with feather disorders do have enough regular physical strength to fly without the aid of feather magic, but long sustained flight is impossible.
The Terran: Terran ponies are able to do earth-magic through their hooves, summoning plants and sometimes animals with their magic. The magical core of the Terran is found within the hoof, partly within the coffin bone of the foot, and partially in the hoof wall itself. Terran magic is surprisingly complicated and it is easy to damage the magical core. Many Terrans in magically-intensive professions use enchanted horse shoes to both protect and enhance their magic. The most expensive horse shoes are actually magnetized, allowing Terrans to pick up metal items with their hooves. A magically disabled Terran is the most well off of the tribes, with terran magic not being necessary for a good quality of life.
The Zebra: Similar to their Terran cousins, Zebras channel their magic through their hooves, and can have the same problems with them. Zebras, however, also have the added power of their stripes, which can magically change similar to a cutie mark. Zebra 'cutie marks' are known to the zebras as their chosen gift, and are often more abstract than their pony cousins. A zebra's entire body can change with their chosen gift, and the most powerful zebras can look like a walking mural. Zebra chosen gifts are also entirely unconnected to pony cutie marks, which emerged nearly 2000 years later.