Character: princess luna
Princess Luna is Celestia's little sister and also an Alicorn. Beginning, presumably, at the time when she and Celestia ascended to the twin thrones of Equestria, she raised/lowered the moon each night, but she became jealous of Celestia because the ponies slept through the night and thus could not appreciate her work. This jealousy eventually transformed Luna into the shadowy Nightmare Moon.
As Nightmare Moon, her body is black in blue armor with her mane and tail appearing to be made entirely out of sparkly indigo, purple-rimmed moondust that she can use as limbs. Following an epic Michael Bay magical lazar battle that ruined the Castle of the Two Sisters (see episode: Princess Twilight Sparkle), Celestia was forced to seal her in the moon after she attempted to bring eternal night to the world. During the first episode, she breaks free after a thousand years and returns to Equestria on the longest day of the year, intent on carrying out her plan again. She uses her powers to hinder Twilight Sparkle and the other ponies in their search for the Elements of Harmony. Eventually, she gets blasted by a rainbow nuke and is subsequently turned back into her original form, which is that of a blue slightly taller than average pony with light blue hair.
According to Lauren Faust, Luna wasn't evil in the first place as she became Nightmare Moon due to being possessed by some outside force which made her change. This notion was expanded upon in the second arc of the main IDW comic series, but it was almost directly contradicted in Princess Twilight Sparkle which takes a more literal interpretation of the story told at the beginning of the first episode in which Luna just goes crazy with jealousy.
Whatever the reasons for Luna's transformation and subsequent exile
bitch who doesn't like bananas, she would undergo more changes as the series progressed. In the Season Two episode Luna Eclipsed, Luna, having regained some of her magical power, appeared considerably taller than when we had last seen her, slightly shorter than Celestia who remains the Almighty Tallest. Luna's mane and tail had also taken on the ethereal quality possessed by those of her big sister, appearing to be a translucent dark blue shimmering field full of stars.
More importantly, and for no discernible reason, her voice changed from the mousey warbling tone it had in The Elements of Harmony into a BOOMING, ECHOING FAUX MEDIEVAL FUS ROH DA in Luna Eclipsed (a reference to an IRL medieval tradition as is Luna's use of the "Royal We"). Obviously, this was awesome, but the untermenschen who had made Cutsie Wootsie Woona their waifu turned it into the first great fandom shitstorm. It should be noted that every version of Luna's voice has been done by the extraordinarily versatile Tabitha St. Germain.
Beyond inciting some excellent chaos, Luna Eclipsed fleshed out the character considerably. It portrayed her as a lonely and slightly deranged royal with an idiomatic affinity for things dark and ghoulish who, quite understandably, was going through a massive culture shock after having missed the entire last millennium.
The Luna Eclipsed version of Luna lives on in the IDW comics, most notably the Luna Micro produced by the Cook/Price/Breckel team. In print she can be seen making loud and frightening declarations, referring to common ponies as "peasants," behaving like an autocrat, and generally getting medieval on Equestria's ass. She has also acquired a pet opossum which she has affectionately dubbed Tiberius, or "Tibbles."
The show itself took Luna in a different, and far more disturbing, direction. After making a token appearance in A Canterlot Wedding in which nobody seemed to realize that she was talking to them, Luna's next big episode was Sleepless In Ponyville in the regrettable third season. Suddenly and inexplicably absent the Royal Canterlot Voice and her medieval mannerisms, Luna took a page out of Freddy Kruger's portfolio and began intruding upon kids' dreams. That's right, the creepy princess that children make candy offerings to so that she will not eat them alive is now making uninvited appearances in their nightmares!
One, two, Luna's coming for you...
Scootaloo does not seem to be put off by her nocturnal visit despite the fact that her only previous experiences with Luna were cowering in fear from her in Friendship Is Magic and running away screaming from her in Luna Eclipsed. The dream walking theme was revived in the episode For Whom the Sweetie Belle Tolls. Sweetie Belle is similarly unaffected. The latter episode saw Luna express her difficulty with constantly being out-shone by her sister.