Princess Luna is my fave FiM char next to Fluttershy, and I like to pretend that that if she were to have a prize pupil/apprentice the way that Princess Celestia has Twilight Sparkle, then it would be my Lunacorn, since they share the same name, have similar cutie marks, and are both moon guardians/moon spirits and "ponies of the night". I can also imagine that Lunacorn might have kept Luna company during those long, lonely days that the Princess was exiled to the moon, and so here they are, hanging out and chatting away like old friends. (More likely, my Luna is probably chatting the Princess' ear off, since, like Fluttershy, she's only shy until she finds someone like-minded to talk to.) ;)

Note: Princess Luna is © HASBRO and Lauren Faust, and both chars are done in my own version of the FiM style, so my apologies if the Princess doesn't look exactly as she does on the show. And yes, I know the moon looks cheesy (literally); it's supposed to. :D Also, the title is Latin for "Moon Friends". :)