If a player asked me to stat up Princess Luna, I would initially be inclined to simply stat her as a god. Give her a symbol (her cutie mark), some domains (night, darkness, magic, good [if Luna]/evil [if Nightmare Moon]), a portfolio (Night), and something like average Deity HD for game world (likely not more than 15-20 for Equestria, but if she's in FR, she should have more) with a couple of special abilities (Commanding Voice, all illusion spells as a 20th level sorcerer).