So, some pretty awesome fanfics have made me partial to the idea that Celestia and Luna started out as earth ponies. So this is what I ended up doodling when I should've been working.
time: absolutely no idea
Edit: Yes, the initial inspiration came from the fanfic "Paradise" www.fimfiction.net/story/273/P…
with a heavy dose of personal theories added in. That fanfic is amazing.