Luna and Nightmare Moon is strangely similar to The Meta and Sigma from Red vs. Only Nightmare Moon is a alternate personality Luna created to cope with her own loneliness (or completely different being in the comics.) while Sigma is an A.
Little Sister :'( this was when Luna was the mare in the moon. i bet she took on twilight sparkle as her student becuse she reminded her of her lil sis and her names twilight-(see what hashbro did there?)i bet thats what she fellt all those years