Luna Headcanon

Go see Celestia here!

Celestia was born to noble unicorn parents, sometime after the unification of the three tribes. Her father is a highly placed member of the team of mages that is responsible for moving the sun and moon. Unfortunately, her mother dies when Celestia is very young. 

While the fire of friendship had theoretically brought everyone together as a harmonious nation, some things don't change that quickly. There was still a strong trend towards tribalism, especially among the unicorn elite. Which is where Luna comes in. 

Luna is Celestia's half sister, born of an affair between their unicorn father, and a bat pony guard. Luna is a bastard, half breed child. 

Her father isn't a bad pony. After the death of Celestia's mother, his loneliness got the better of him and he fell for the mysterious, romantic thestral mare. Relationships between bat ponies and other tribes are rare, and almost never produce children, so Luna came as a surprise to both of them. But there's a problem. Luna is a unicorn. Bat ponies at this time are almost exclusively confined to military roles. Her mother is unable to raise her, and without wings there would be no place for Luna in the bat pony guard troupe anyway. So, not wanting to abandon his second daughter to a harsh, short life, her father takes her in. 

Celestia, after some initial shock, falls head over heels for her baby sister. She's too young to really understand why people seem to sneer at daddy now, or why everyone is always mean to Luna. But as the two fillies grow, they are inseparable. 

Luna isn't exactly welcomed by polite society. As she grows, she constantly tries to live up to her perfect older sister, but she can never quite seem to be good enough. Her tufted ears, un-cloven hooves, and slit pupiled eyes mark her as an outcast, even to those who don't know her. As time goes on, she develops a growing inferiority complex, and a craving for positive attention. Celestia is a great sister, but she just doesn't seem to get how bad things are sometimes. 

When both sisters ascend, raising the sun and moon by themselves for the first time in history, no one knows quite what to make of Luna. She is extremely grateful that becoming an alicorn didn't give her bat wings, but she still looks a bit too different for most ponies to be comfortable with. Over time, as her resentment slowly builds, she tries to suppress the more intimidating aspects of her appearance. When she finally snaps, her magic runs wild, and all of the predatory bat pony traits come out in full force. Nightmare moon is the result. (I will be drawing NMM soon, don't worry.)

After her return to Equestria, Luna is much more relaxed and open. Her time as Nightmare had lasting effects on her appearance, especially her mane. She has also kept some of the spines and markings of Nightmare's more intimidating form.