Princess Luna

Princess Luna is the Princess of the night. She is an Alicorn with a midnight blue coat and a shimmering blue mane. Her cutie mark is the moon in a night sky.

Derpy Hooves is the smartest Pegasi in Equestria. She was sent undercover to Ponyville by Princess Luna and Princess Celestia.

Princess Luna's transformation into Nightmare Moon was caused by her anger about Princess Celestia's decision to reduce the night's length to the timespan where really everypony was sleeping.

Princess Celestia and Princess Luna unleashed the Windigos upon Equestria to prevent Ponies from warring.

Princess Luna's birth date is the winter solstice. Princess Celestia's birth date is the summer solstice.

Nightmare Moon is actually her normal form. Princess Luna is her "powered down" form due to the use of the Elements of Harmony.