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Equines appear to be the dominant species in that universe. Not too big of a stretch.
Okay, everypony! Get ready for a headcannon artillery strike!
Unlike other ponies, Celestia and Luna were born with their Cutie Marks.
They
are the goddesses of the Sun and Moon, right?
All creatures in Equestria are inundated with varying amounts of magical energy
That would explain how Cutie Marks appear as if by magic, and how flying creatures can walk on clouds.
Star-Swirl the Bearded was Twilight's great-great-great-great grandfather.
He was a great Unicorn Mage, and Twilight shows the potential to become a powerful sorceress. It's possible that she is of Star-Swirl's bloodline.
There are Sea Ponies somewhere in Equestria
...We just haven't seen them yet...
I saw a very sad PMV called Exile that featured the song
Exile Vilify from
Portal 2. The video was about Luna's corruption and transformation into Nightmare Moon, and the great emotional trauma experienced by both her and Celestia (but then again, what else would PMVs based on those events focus on?). The song matched the mood oh-so perfectly...It made the ending of
On a Cross and Arrow pale in comparison.
You've got sucker's luck
Have you given up?
Does it feel like a trial?
Does it trouble your mind
The way you trouble mine?
Exile
It takes your mind again
-sniffle- So very sad...it's the kind of song that plays when you're feeling hopeless and alone...