Long before Equestria was even a thought, a good majority of the world was once ruled by the deer. They were able to do this because of their abilities to predict the future and their cunning. Given that there weren't any large countries or anything of the sort back then to oppose them, their ascension to domination of the planet was secure.

The history of the deer before this rise to supremacy is scant. Like most sapient races alive at this point in time, they were once a tribal people. Wars between different tribes and with other species were frequent. Archaeologists have found evidence of these inter tribal wars with bones being punctured or broken with tools the deer used at the time and other pieces of evidence. The deer don't deny this and we still see these inter city conflicts to this day, though rarely to any deadly degree.

However, their inter tribal conflicts were ended when the tribes were confronted by what scholars and the deer consider the first Farseers. Given that the Farseers are secretive about many things, even to their own people, they have confirmed the age in which this happened. Namely, about 75,000 years ago, just five thousand years after 'The Great Unification' where the other realms were 'merged' with our own. Evolution and other features of species were warped and changed and it appears that amongst some deer, this took on the form of powerful magic and future sight. Said powers of a farseer aren't found in other species, aside from the gods themselves.

Within a few centuries to a thousand years after their appearance, the deer were quickly unified through the wisdom of the Farseers (and a few inter-species wars) before turning themselves on the rest of the world. They were largely successful and between 74,000 to 41,000 they ruled the known world.

What changed? For thousands of years, no one knew exactly. The records of that time period are almost non-existent outside of the deer's own. The earliest known recorded scriptures are those of by the Snek which only date back to about 28,000 years. The only possible answers anyone had was that of legends passed down by generations, with variations telling it differently. (The religion of Faust took influences from this as well)

Piecing them together, we can get a picture of a centaur creature that could absorb the energy of others destroying what the deer had built. This left others scratching their heads on if this was true or not.

That is, until the fabled human, Megan, slew the beast herself.

Coming clean and sharing records of the events with Megan, we now know the truth. It would seem that the deer grew complacent in their rule. They grew to throw themselves into every avenue of pleasure and knowledgeable exploit just to entertain themselves. They had the world at their hooves after all and had that for thousands of years. So taking into account the realms, they decided to use one realm that they hadn't mastered before.

The Realm of Darkness.

The deer believed they could control the forces of this realm and to an extent, they could. It has been proven that control of that realm can be done in small doses. However, there hasn’t been much study of the realm due to the corrupting nature of Dark Magic and the havoc it plays on other beings. While chaos is damaging in its own ways, there are ways of controlling those powers easily enough. Dark Magic on the other hoof relies too much on negative emotions and distorting them to an unfavorable degree.

The deer found this out the hard way. Soon, many of the city states practicing the Dark Arts became corrupted and become what we now know as the Shadow Deer. The Farseers, seeing their impending doom in the hooves of these Shadow Deer, struck with the other non-corrupted city states. This war went on and on as both sides had Farseers in their employ. However, the Shadow Deer were losing.

On the cusp of defeat, they gave many souls to create something that could win the war. Unfortunately, what they created was a monster that had no allegiances. All he wanted was power.

His name was none other than Tirek.

The dark centaur drained many deer, using their power to grow larger and stronger. He ravaged the rest of the Deer Empire. Finally, several Farseers gave their lives to seal him away. (Until his eventual escape and death at the hands of Megan.) The Shadow Deer went into hiding in the Dark City, a place they only leave to pirate others and take slaves to satisfy their dark endeavors. The deer fell into ruin and they never recovered. They fell into infighting and desperate attempts to keep their numbers going.

Unfortunately for them, other races stepped in to fill their shoes and they couldn’t become the super powers they once were especially now that they were divided themselves.

Because of their thousands years of rule, they are very haughty and full of themselves as they believe themselves to be the best sapient to have ever lived and boasted about having the first empire. Some city states still harbor a wish to rebuild this world wide empire and conspire to make it happen. Others have isolated themselves. Some have allied themselves to other factions in order to keep themselves safe and also the world.

Nice peice of history. Very interesting. I also like how you're making some races sentient in your 'verse that don't appear to be either sentient, or even exsist, in cannon. These Deer are a prime example. I remember seeing a deer in one of the cannon episodes, but he was being given away as a pet, so I don't think the deer are sentient in cannon.