Ever have one of those days were the stuff you draw just makes you happy cuz you like how it turned out? Well this is mine for today.
My Headcanon: As alicorns grow older a/o or powerful they begin to change in physical appearance, usually to suit a theme or physicality that is geared towards their domain. This may be very gradual or incredibly abrupt; regardless it is inevitable.
Celestia, House Sol, Empress of Equestria: Whether or not Celestia's appearance was onset quickly or slowly is unknown, as she has had the power to keep it hidden from her subject for a very long time. Presently, in 2017 A.T.S., she has taken advantage of her country's steadily growing acceptance of varying appearnaces and mentailities to progressively reveal her natural appearance. More draconic than afore, she displays traits of both reptile and bird, bearing resemblance to both dragon and phoenix. Scales and yellow tinted markings line her neck and underbelly. Her canines have since grown a bit longer, though not much sharper. Her feathers and hair beginning to flare out more and take on warmer, more fire-like colors in stead of her normal blue, green, purple and magenta. Her new appearance has received some stigma, but mostly from other countries, rather than the subjects of her own.
Luna, House Noctis, Empress of Nocturnia: Unlike her sister, Luna has appeared mostly as seen here nearly her whole life. Her more nocturnal features (large tufted ears, slitted pupils, fangs, bat-wings) have been present since birth and have not undergone many major changes since then, other than mane color and size.
Mi'Amore Cadenza, House Sors, Queen of the Crystal Empire: Unassuming to most, Princess Cadence's appearance is not often talked about as something that is so odd or unusual. Perhaps this is because her appearance is not the same to everyone. Aside from the few standard variables that make Cadence, Cadence (pink coat, purple-yellow-magenta mane, crystal collar, purple eyes) her general facial and physical features may vary slightly from creature to creature. Her appearance is ever-changing in the eyes of everyone (with the exception of her husband, Shining Armor and her son Diamond song, both who could only see her as she was born/destined to appear). In a sort of Aphrodite way, she appeals to all, even by taking of more masculine features and sounding slightly different from ear to ear.
Twilight Sparkle, Clan Sparkle, House Concordia, Sovereign leader of the North Star Nation: Twilight, being the newest ascended alicorn on the mortal plane, was in for a wild ride as her new life as an immortal entity began, particularly because her training in alcornic magic had to be cut short during the Second Great War. And as such, she was not aware that her appearance would change so drastically (and so suddenly). While in hiding with her subjects in what would become to be known as the North Star Nation, Twilight found the hidden tribes of Mammoths. With their discovery, also came the discovery of new and mysterious magics. Upon learning these magics more closely, Twilight's power grew beyond her current form, and with such a growth spurt, egged on the drastic alteration of her appearance. Nearly overnight she became what we see here — a five eyed pony (not to mention the horn and the mane). Each eye represents an Element of Harmony (hence their irises matching the other five Element's coat colors). Twilight became a sort of religious figure amongst the mammoths, they called her The Rainbow Serpent, or simply Rainbow Sparkle to better acclimate her to the pedestal they had put her on. The reason they revered her so was due to their ancient tales of their god, The Rainbow Serpent, who bore every color of the rainbow on her long sleek body and saw over the world of the living with her five eyes.
But what about Twilight's Element? Shouldn't there be six eyes for each element? Well yes, and she does technically have a sixth, but it her "Mind's Eye" (what the mammoth's refer to one's connection to magic). Anyhow, her horn seems to flow with the all the colors of the spectrum and the ends of her mane and tail mimic what the sky looks like at that time of the day (depending on where she is).