Here's a HP/MLP idea that I've had bouncing around and finally got around to writing down so enjoy
Iris Rose Potter sighed as she counted down to midnight and the start of her 7th birthday something that she did every year as her aunt and uncle refused to celebrate her birthday.
"Happy birthday to me" Iris says as the cheap digital watch she had grabbed from Dudley's second room hit midnight and Iris made a wish.
"I wish for a friend that Dudley can't chase away and will always be with me" Iris says as she covers herself up in the blanket that her aunt had given her because Dudley didn't like the colour of it, though sometimes Iris thought her aunt Petunia bought things in colours she knew Dudley didn't like just to give her new things on the sly.
Nightmare Moon paced around where the Elements of Harmony had locked her away at when she paused at the feeling of magic locking onto her before she her though the magic a wish from what sounded like a young filly.
"Hmm" Nightmare Moon hums to herself as she thought about the implications of the magic and the wish it carried as she sat tapping a hoof to her chin.
Nightmare Moons choices was ignore the magic and the wish it carried as it was waiting for her to choose what to do or to follow the magic to the young filly who was wishing for a friend and if the wish was anything to go by then she would most likely end up also doubling as the young filly's mother.
"Oh who am I kidding anywhere is better then this boring place that the elements has stuck me in" Nightmare Moon says before she lets the magic and the wish it's carrying grab her and take her away to the young filly who made the wish.
Nightmare Moon blinked when she arrived at where the magic and wish brought her to and was surprised to see it was a human filly who had made the wish seeing as humans had left Equestria when her light and naïve side Princess Luna had been a young filly as despite what Luna or Celestia thought Nightmare Moon was just Luna's repressed dark emotions and thoughts given it's own personality but now she had a young filly to look after and raise.
It was morning when Iris woke up and yawned before blinking when she spotted a unicorn with wings floating near her causing her to say "Beautiful".
"Thank you young filly my name is Nightmare Moon I'm a Alicorn and I have to ask why is such a young filly such as yourself sleeping in such a small and cramped location?" Nightmare Moon says to the now awake child.
"Umm I'm Iris Rose Potter miss Nightmare Moon" Iris says smiling brightly at the dark alicorn before her smile fades and saying softly "And I sleep here because it's the only place my aunt and uncle will let me sleep" causing Nightmare Moon to go wide eyed as the very thought someone would force a filly to sleep in such a place is appalling to Nightmare Moon.
So short opening chapter but I hope you enjoyed reading it