Oooookay. I've been working on a headcannon for awhile now. It goes along with my unimule character, Artsy.

I was thinking the other day: In traditional lore, unicorns were often seen and portrayed as large horses with cloven feet and long tails, tipped with hair.

In the world of MLP, it would seem that only ponies themselves can be unicorns, which kind of interested me. When the horses from Saddle Arabia showed up at the end of Magic Duel, I couldn't help but do two things. Laugh hysterically at how tiny ponies were in comparison, then wonder about unicorns, like a always do. Do they have unicorn horses in the MLP universe? I have no idea. Then I got to thinking about donkeys and mules... you can see where this is going.

The headcannon I have devised for myself not for the entire MLP fandom, believe what you want, is that unicorns come in all species. The first unicorns where in fact donkeys and mules, letting the medieval showing of them come forth here. They didn't have cloven feet though (even though I adore cloven footed unicorns \*-*/) they were just common equine. Eventually, the first unicorns, the donkey unicorns, began to die out. In an effort to save the magic the possessed, the donkeys bred with horses and ponies, creating the first actually accepted mules and unimules. The unimules thrived for centuries, due to the strange fact that they were not sterile.

A few thousand years passed, and Discord came to rule the lands. He managed to screw with the unimule gene just enough to make it hard for them to reproduce. The royal sisters, who came into rule just at the tail end of the century old reign of Discord, attempted to fix the damage, but, it was too late. Unimules were dying out.

They tried the same self preservation technique that their ancestors had used, turned yet again to horses and ponies. This time, however, no foals could be produced. They either didn't conceive, miscarried, or where stillborn. The unimules, in a last ditch attempt to save their magic and lore, turned to the royal sisters, who agreed to bestow horns to foals while in the womb. the first horse and pony unicorns were created.

The unimules continued to dwindle, though managed to keep their kind alive. More and more normal mules were born however. Eventually, the year that Princess Luna became Nightmare moon, all unimules were rendered completely sterile, due to the magical increase the princesses created while in battle. Their horns and magic didn't know how to deal with the increase, so, in a last ditch effort to keep the individual equine alive, the horn channeled the magic to many parts of the body, causing sterility in most of the mules.

And so, unimules all but died out, only one or two being born in each century, created by a unicorn pony or horse, and a donkey.

(in a nutshell: Unimules were some of the first unicorns, Discord came and screwed things up, unimules gave their magic to ponies, and the unimules pretty much died out)

So, yeah. That's my headcannon. Disagree with it if you want, it's still how I think about MLP universe now XD I'm sure I left some historical holes in there, but, I wasn't giving a college lecture. It was just a quite jotting down of my ideas :3

I know, it was a long read. If you didn't read it, feel free to put a TLDR in the comments lawl.