No, I'm not going to be running MLP as a D&D game, only adapting the concept, or at least parts of it.
I'm using elves as the equivalent of ponies, because their default fluff is kinda similar to pony fluff, except maybe their alignment (ponies live in harmony and order, elves are generally Chaotic).
Pegasi are Winged (SS) elves.
Earth ponies are Feral (SS) elves.
I don't know what template to use on an elf to make a Unicorn. It would have to be something magic related. Improving mental stats, but not physical (can decrease the later, but not by too much). Preferably something that gives minor magical abilities (don't really have to mimic Unicorn magic, this game world will be only loosely based on MLP). Horn isn't important, that can be fluff only. Preferably something that changes Humanoid into Monstrous Humanoid (but only because Feral and Winged do too
).