In my headcanon, Twilight is a hereditary unicorn, meaning both of her parents are unicorns. A true, or “pure” unicorn would be a unicorn in which no other pony species’ DNA has merged into their bloodline. For example, Applejack is a true earth pony. There is not a single trace of unicorn or pegasus blood in her family’s entire history, that is, until she married Rarity. Twilight’s father, Night Light, is half unicorn, half bat pony (meaning one of his parents was a bat pony, a rare subspecies of pegasus). While the observable bat pony traits skipped her father, they are very evident in Twilight.

Twi has sharpened canines, an overall fluffier pelt, slitted pupils, and sharper ears. All of these traits are commonly associated with bat ponies, and as an alicorn, all of these features become heavily accented. She is absolutely a “creature of the night.” Sometimes, her eyes even glow in the darkness.

Celestia’s original plan was to have an heir to the sun and an heir to the moon should anything happen to her or her sister Luna, or if they just wanted to retire. She sought out Sunset Shimmer, who has an extremely rare sun affinity and showed considerable magical potential from a very young age. I’ve mentioned this before, but every pony has their own affinity. Some can even have two: Rainbow Dash has rainbows and sonic booms, Fluttershy has a nature affinity, Pinkie has a chaos affinity. Some of the rarest ones are elemental affinities and celestial affinities: Sun, moon, water, and fire. (Air and earth are not as rare, given that most pegasi have a connection to air magic, giving them their ability of flight, and most earth ponies have a connection to the earth).

Twilight’s affinity is magic. Pure, raw, unadulterated magic. But, like Rainbow Dash, she also has a second affinity, and that is the moon. One could call it her “lesser” affinity, since it’s definitely not as obvious, and she doesn’t even know about it until way after she becomes an alicorn. But it’s absolutely within her power to raise and lower the moon just like Luna does.

Celestia’s plan is set: she has an heir to the sun (Sunset Shimmer), and an heir for the moon, Twilight. But it all goes terribly wrong when Sunset proves to be more power-hungry than she ever expected. So she banishes Sunset, thus losing her heir. She panics, and chooses the promising pegasus Cadenza as her next heir. Cadence far exceeds what Celestia ever hoped her to be, going on to restore love in the Crystal Empire. But she’s still technically the next heir to the sun.

Like Celestia and Luna, Cadence and Twilight are said to be the next celestial “sisters.”

Luna will eventually teach Twilight how to dreamwalk, an incredibly draining but powerful form of lunar magic.