Ok, here is mine. It is a biggie, so sit patiently.
Also, this may contain small Equestria Girls spoilers. If you have not seen the movie and do not want slight spoilers, then do not read any farther.
So, in Equestria Girls, when Twilight enters through the portal into the other world, she emerges in a human form. The same thing happens to the villain, Sunset Shimmer. Now it could easily be said that their is a magic hat turns the ponies into humans and vice versa when they enter through the portal, but I have a different idea.
Now, first we must look at the comprehensive guide to the multiverse, Magic: The Gathering. In MTG, humans seem to be the most common species in the multiverse. Now many variations could exist between the human races, such as how the Vulshok have metal growing from their bodies, so the people in Equestria Girls can easily be classified as human.Now, as we all know, there are no humans in Equestria. We have never laid eyes on a human, and the well read Twilight Sparkle has no idea what she is looking at when she sees a human. But also, we see no other humanoid races in Equestria, such as elves, goblims or kithkin which are also prevailent in the multiverse. Furthermore, in most planes in the multiverse, ponies and other creatures are nothing more than beasts, with no more than an animal level of intellect. Now it could easily be said that since the multiverse is infinite, there could easily be a plane of existence where there are no humanoid life forms and that ponies have developed sentience. But that does not explain why the ponies transform into humans and back again when going through the portal.
My theory is that there is a magic in Equestria that transforms what would have been the native human population into ponies. This would explain why Twilight transforms into a human when entering the other world. The magic is no longer in effect there and she simply reverts to what would be her true form. Now, who was the one who put this curse on the ponies? My theory is Discord.
Now, let us look at Discord first. He is a chaotic being, doing random things just for his own amusement and for the pain of others. It could easily be believed that Discord would turn an entire population into colourful pastel ponies just for amusement. Now, when Celestia and Luna, newly turned into ponies, used the Elements of Harmony to defeat Discord, they were turned into Alicorns by the Elements, same as how they transformed Twilight. The curse, already in its place, could not be lifted, and so the ponies decided to live on their lives and not tell their descendants of who they once were, not wanting to build up hopes or let them know of a life they could never live. This also explains many human elements that exist in Equestria, such as objects that would normally require hands instead of objects existing that would adapt to hooves.
Now, why are some ponies unicorns, others Earth, and some pegasi? Some may point towards ethnic origins, but I actually believed it was social class. The differences between the races shown in the Hearth's Warming Eve episode, I noticed similarities between the races and the social classes made by Plato in The Republic. The unicorns are the Guardians, the high class and well known philosophers and leaders that led the people. This is why the current unicorns live in Canterlot, and received magical abilities to show they are a step above the rest. The pegasi were the soldier class, which is why the pegasi were shown in armour and military garb in the past. As well with flight making them more efficient soldiers and physical guardians. The Earth ponies were the working class. That is why they semmed the poorest, and also why most earth ponies we see are workers, such as farmers or bakers. Also, according to Plato, the parents of a person do not decide the class of the person themselves, which is how a pegsus could be born to two unicorns.
Now the different races arguing and going their seperate ways in Herath's Warming Eve is just as Plato described when the different social classes stagger and fight and the perfect government falls to lower and lower forms of government. This eventually reaches a monarchy, with Princesses Celestia and Luna, the people's heroes after defeating Discord, were promoted to the monarchs. Now the fractioning of the social classes happens before Discord's attack, but since modern ponies do not know what a human is, they assume that these people were ponies just like themselves.
Whew, that was quite a bit. Let me know what you think and if there is something that needs more explaining or some more work.
It's not just about the cards. It's about putting a little bit of your heart into something that you care about.