"Way way back, many millennia ago...", Twilight started. " A beautiful alicorn named Queen Galaxia lived with her husband, King Cosmos as the highest rulers of what had recently become Equestria. They were happy with how everything was going. The pegasi, unicorns, and earth ponies were cooperating with one another successfully, and without conflict. Three weeks later, Galaxia had shocking news for her husband. "I'm pregnant, we're going to have a foal", she said to him. He smiled warmly, and kissed her happily. Nine months later, after loads of preparation for their new baby, Galaxia went into labor, and Cosmos rushed her to the hospital. She was in pain for a while, but they cried tears of happiness when they heard the first cry of life from their little one. She had a horn, and tiny wings on her soft white back. Her beautiful mane was pastel, and multicolored. It is unknown what they named her. Five years later, Galaxia told her daughter that she was going to be a big sister. The filly jumped for joy and kissed her mother. After nine months, she had tears of happiness in her eyes as she cradled her newborn sister in her hooves. Her sister was the color of night, and had a bluish-violet mane. The filly gave her sister the name of ... Luna."
Twilight and her friends were wide eyed.
"They were alicorns at birth?", Rainbow wondered aloud.
"There's more to it.", Twilight said. "Many years later, an alicorn princess who was married to a unicorn, lived far north in the early Crystal Empire. She soon gave birth to a filly, who looked like a unicorn at first glance, but soon sprouted wings just two minutes later. The filly was named Mi Amore Cadenza, and she became the first princess who had the power to spread love. From then on, the legend has been that if an alicorn princess is married to a unicorn, there is a 50/50 chance that each child they have, will be an alicorn at birth.
They also found two scrolls on a shelf. Twilight did not know what they were, but soon found out when Dash unrolled and examined them.
"These are Celestia and Luna's birth certificates.", said Rainbow, when she saw their parents' names written, and the pictures of Celestia and Luna as newborn fillies, sound asleep and wrapped in baby blankets.
"I can't believe it!!", exclaimed Twilight. "Celestia and Luna were born as alicorns, and Cadence became an alicorn two minutes after her birth."