Crazy useless MLP:FIM theory number 1:
In the future, there is a nuclear holocaust.
Almost nobody survives.
Except a few people who evolve.
The evolve into horse like beings with mutated appendages.
Horns = massive brain tumors
Wings = Strange mutation
Magic = Mutant psychic powers
They also put an atmosphere on the moon or something which is why Luna didn't die instantly.
Also: the laws of physics stopped working (god help us) because of the nukes and that's why the earth/moon stopped spinning.
Swords (which ponies have no use for, due to them not having hands)
Saddles (useless without humans, unless sex)
The parodies of major cities
Why they all speak English (do they, or is it translated)?
Why they have the same star patterns (orion is mentioned)
Why it takes place on a planet that's Earth
Why there are no humans
This is confirmed as Adventure Time's setting, so why the hell not.
PROVE ME WRONG INTERNET.