Even ponies don't dream solely about happy things. Princess Luna sure could tell countless stories about the bigger and smaller nightmares that are haunting ponies' minds. Especially the most powerful nightmares often have their roots in buried memories, or memories that one wants to forget forever.
I imagine Starlight Glimmer being such a haunted pony. No matter if the folks in her old village have forgiven her, no matter how her new friends think of her now, those old unwanted memories sure would surface now and then… especially with everypony constantly reminding her of her past.
Another one of my dreaming ponies. It hurts me physically to have drawn her in such a miserable state, but I wanted to draw a pony having some kind of nightmare, and poor Starlight just got the short straw I guess. Here's my promise to Starlight: The next time I draw you, I'll make up for it and draw you being really happy! Pinkie Promise :)
I've used very few pre-made vectors as resources this time, did most of the work myself! (that's why it took so long and looks so bad =P )
This is mostly because I couldn't find any good vectored backgrounds of either Starlight's room or the cutie mark vault cave, so I re-drew them completely using screenshots from out of the show as reference. (The cave being a veeeerrry free interpretation… hey, it's a dream!)
I also used this opportunity to have some fun with the bookshelf :D