They call me "alicorn." It's an odd name, but fitting in its way.
If you look in the dictionary, you will find two definitions for the word. The first, and most common, is that it refers to winged Unicorns. Specifically, two of them: my sister and I. Genetics and chance do occasionally result in other Unicorns with vestigial or functional wings, like my dear niece Cadance, but they are a different breed, and the word is rarely applied to them.
The second definition, now rarely used outside of medical fields, refers to the material of a Unicorn's horn. This definition is much older, and it is much more closely related to the first than most ponies think. Allow me to explain.
My sister and I were born different. It was only 150 years after Equestria's founding, and marriage between the the tribes was still quite rare. Our understanding of the genetics of that sort of interbreeding, and of genetics in general, was very limited. So you can imagine the surprise of our Earth Pony parents when our mother gave birth to a winged unicorn, and then only a few years later, a second.
Our parents, may their souls be forever at peace in the Wondrous Elsewhere, raised us with all of the love and caring two foals could possibly wish for. I will not say that we had an easy childhood, for there was always work to be done, but it was a beautiful one, and we learned all of the virtues Earth Ponies hold dear: honesty, tenacity, joy, and humility.
But we were not only Earth Ponies, and we were never limited in exploring our mixed heritage. Shortly after my tenth birthday, and my sister's eighth, I began to take an interest in the work of the Pegasus Ponies who flew overhead and arranged the clouds and the weather. One day I called one down, and we spoke at length about the sky and the air and the clouds. He quickly became a dear friend to both my sister and myself, and we learned from him the character of the Pegasi: courageous and free-spirited, loyal and kind. And from the day we met him, our farm had the most perfect weather in the entire land, for he also taught us how to move the clouds and bring the rain.
But neither were we only Pegasi. By my fourteenth birthday, I had still not found my special talent: the one ability with which I would forever define myself, and which would bring forth what we now call a Cutie Mark. In search of answers, my sister and I bid a fond farewell to our beloved parents, full of promises to return one day with our heads held high and our souls and flanks emblazoned with new purpose, and set off to the Unicorn city of Canterlot. There we learned of the spirit of the Unicorn Ponies: generosity and grace, nobility and magic. And there we finally found our special talents.
It had ever been, since long before the founding of Equestria, the duty and privilege of the Unicorn Ponies to use their unique magic to bring about the Day and Night. Seeking to learn more of our roots, we joined those who had already taken up that sacred mantle: myself in the Ministry of the Sun, and my sister in the Lunar Council. We were greeted with curiosity and some uncertainty at first due to our uniqueness, but we were never shown hostility for it, and we soon proved ourselves as capable as any of our peers, earning our places and cementing our friendships there.
It was at our first Summer Solstice as members of the Ministry and Council that the moment we had been waiting for finally arrived. We had each been chosen to lead our respective groups in the Grand Rising that year, and as we stood facing each other on the great dais, we felt an energy in the air that was much more than the sum of the Unicorn magic we were guiding. The Sun and Moon welcomed our touches, almost cried out for them, and the stage we stood upon was bathed in their light as they traded places in the ancient ritual.
The gasps and murmurs of our audience told us that something unusual was happening, and it was followed by an awed silence as the light faded. We stood before them as more than ponies in that moment, still clothed in the power of our celestial charges, and even the unicorns who had stood with us watched us with a surprised reverence, the magic they had never had the chance to cast still glowing on their horns.
Then the spell was broken by a soft sound like the tinkling of glass chimes, and our flanks lit up with the appearance of the symbols we wear to this very day: the emblems of the Sun and Moon. A great cheer went up all around us, and the solemnity of the ritual was forgotten as friends from both on the stage and off it swarmed us with embraces and congratulations.
Naturally, the first thing we did as soon as the festival ended was take to the skies and fly as fast as we could back to our old home to give our parents the news. There seems to be an unwritten rule of narratives like this one which states that, having achieved their greatest goal, the joy of this wonderful moment must be shattered when the heroes return home to find that their old homestead is no more, or that their parents had passed away in their absence, or that they had arrived just in time to hear their most likely trite and cliche dying words. I have never seen the appeal in such cruel twists of fate, and I can assure you that it was not so with us.
Our home was just as we had left it, other than that it had been spring then, and our parents, as hale and hearty as ever, welcomed us in with open arms. We babbled excitedly about our time in Canterlot over a delicious Earth Pony dinner of baked potatoes and corn, and our parents beamed with pride at the mares their two daughters had become. We spent the night and left the next morning, after a great many more hugs and nuzzles and promises to write.
The years in Canterlot passed quickly after that. We soon became the leaders of our respective councils, and over time the sizes of the groups began to dwindle. Not because of any disharmony or lack of interest, but because as ponies retired or moved on to take other jobs, there was simply no need to replace them. Where a score of Unicorns had previously grouped together their magic in a clumsy effort to force the Sun and Moon into their places, my sister's and my own mastery and growing power allowed us to do it with less and less assistance, and then finally, alone.
As the years passed, we became increasingly aware of how different we truly were from other ponies. Where others aged, we simply grew stronger. Even as we welcomed our elderly parents into our Canterlot home to spend their twilight years in well-deserved peace and comfort, we looked hardly a day older than we had upon our first visit to them those many years ago.
They passed peacefully some years later, and it was as happy a goodbye as there has ever been. Even as we ushered them through the final veil into the Wondrous Elsewhere, there was nothing but joy in their hearts at the honest life they had lived and the two daughters they had raised. And we shared their joy, and we knew that they would live on in the love they taught us, as we passed that same love on to all the little ponies who were increasingly beginning to look to us as figures of respect and authority.
Soon after that, we began to change with age again. But instead of becoming weak, our bones growing brittle and our sight going dim, we seemed to grow with the faith in us which grew among our ponies. Each aspect of our heritage strengthened: our horns became longer and our magic more powerful, our wings spread wider and our flight became more graceful, and our bodies grew larger and stronger, until we stood at least a head over any other ponies. We became the living ideals of female beauty, growing to become the noble and almost divine leaders our ponies envisioned us as. All the while, we showered our little ponies with all of our own love and affection, and Equestria prospered. We were each given the title of Princess, to signify the high status we had grown into.
The term "alicorn" refers to the material of a Unicorn's horn, and it is that material which focuses and shapes a Unicorn's unique magic. But Unicorn Ponies are not the only ponies with magic, and they are not the only ponies with alicorn. A Pegasus Pony's alicorn is in the bones of her wings, allowing them to push her far faster than they could without magic, and in her hooves, with which she manipulates the clouds. An Earth Pony's alicorn is in the bones of her body, giving her a near-unbreakable strength and resilience.
But my sister and I are all of those things, and something more besides. As we came into our power, as we exercised each aspect of our heritage, our alicorn grew and strengthened and expanded, until we became something which was both all and none of the three.
We became alicorn.