There is a subject much debated by Equestrian scholars and historians.
Equestrian scholars and historians argue over over a lot of things, though. Whether Sombra was secretly left-hooved, whether Scorpan actively participated in Tirek's assault on Equestria or was just dragged along by his big brother, even whether or not Starswirl the Bearded was secretly a mare wearing a fake beard (Although Starswirl is around now, he refuses to confirm or deny these rumors for reasons that he also refuses to discuss). But these debates, naturally, are ultimately unimportant in the grand scheme of things. There is, however, one subject of debate that could be seen as world-defining for the ponies of Equestria...
What first caused the division between Princess Celestia and Princess Luna?
Scholars argue over certain journal entries, historical events, etc., but nothing concrete can be discovered to have caused Celestia and Luna, sisters so close as to be practically joined at the hip, to experience such a sudden and pronounced divide, one ultimately causing Luna to become Nightmare Moon, and suddenly try to assassinate her sister.
The truth is, however, it wasn't a major groundbreaking event, or even an incident noteworthy enough to enter into either princess' diary. No, it was a moment so simple, so seemingly insignificant, that even the princesses did not recognize it for what it was, not even a thousand years later.
It was the gift of a single flower.
A young earth pony filly, Dew Drop, fresh from having earned her cutie mark, had managed to successfully cultivate the flower now known as Heart's Desire. Until this moment, it had proven impossible for this rare plant to grow anywhere save in the wild. But thanks to the determination of this dedicated young pony, the rare magical flower would soon be cultivated in greenhouses across the land, allowing for potion-brewers across Equestria to be able to make ready use of its fantastic properties.
But at the time , she only had one sample of the fantastic blossom, and the Centennial Festival, the celebration of a century of alicorn rule, was only a day away. She wished for this magnificent flower to be a gift to the alicorns, as thanks for bringing peace to Equestria. However, there were two princesses, and only one flower. Thus, Dew Drop had a decision to make: Which princess would receive the flower?
After several hours of thought, she decided to let the flip of a coin decide for her.
Now, most individuals do not realize it, but flipping a coin is a very powerful magical ritual, one that splits time into three separate paths: Heads, Tails, and the ever elusive Edge. Three separate paths, three unique timelines where each possible result of the coin toss plays out. In the timeline that we all know, the coin came up heads, and the flower went to Celestia. Because of this, Luna felt the first stirring of jealousy for her sister, and... well, you know the rest: Nightmare Moon, a thousand years of banishment, and then Twilight Sparkle and her friends save the day. Et cetra, et cetra, and so forth.
But what about the other two timelines?
In the timeline of Edge, Dewdrop took this as a sign to make the flower a gift to both princesses. Having done so, both princesses took care of the flower together, and grew closer as a result. This timeline ended up being very boring, with nothing really interesting happening since it had both alicorns working together to keep Equestria safe for a thousand years. Since it is boring, we're not going to discuss the Edge timeline any further.
Instead, we'll discuss the flipside of the coin...
In another timeline, the coin landed on Tails, and the flower was given to Luna. And for the first time in her life, Celestia felt a terrible thought pass through her mind: "If Luna wasn't around, I wouldn't have to share anything with her. Everything would be mine. The love of my subjects, the treasures of the kingdom, everything..." That thought grew and grew, until it became Daybreaker. The result was much the same in this timeline as it was the other: Luna used the Elements of Harmony to seal Daybreaker inside the sun for a thousand years. Luna was left, on her own, to manage Equestria, and to her credit, she did about as good a job running it as Celestia did in the Heads timeline.
Changing one thing a thousand years ago, however, can change everything in the present day. Ponies who fell in love and had children in one timeline never even met in another. Ponies who might have changed history were never born, or ponies who never existed in one timeline were born in this one, and changed history. Twilight Sparkle and her friends, as a result of one coin toss a thousand years ago, were never born at all. But nature abhors a vacuum, so other ponies would eventually rise to fill the gap. The Butterfly Effect can be a cruel and terrible mistress, or a fine and classy lady, depending on her mood, and your point of view. Regardless, the flow of causality had been diverted to another stream, and Equestria, if not the entire world, would be a very different place as time went by.
And now, a thousand years have passed, and events a thousand years in the making are starting to come to pass. And surprisingly, the first event that would shake the foundations of Equestria would not begin in Canterlot, or even Equestria, but in Kludge Town, where a homeless, orphaned filly miles and miles from anypony else of her kind wondered what it would be like to feel happiness...