A little history of Equestria :
Back when Equestria was first founded, with a strong burst of magic to scare away the Wendigos, the elements of harmony were first created, six powerful spirits to be the guardians of Ponykind.

Pony society advanced  through the centuries. More and more technologies developed, and soon the unicorns had become scribes, artistans, craftworker, so on and so forth. They no longer had the time and energy to handle the sun and the moon. At the same time, political unrest was rising. It was clear : The ponies needed a leader, and they needed someone who could handle the sun and the moon . The Elements concerted amongst themselves, and chose a noble mare. Celestia, as was her name, took pride in accepting the burden, and became the first Eternal Princess of Equestria.

Centuries passed, and Equestria prospered, but soon, signs of unrest began to appear.  The ponies were worried for their leader. She was tired, she was exhausted. The burden of carrying the sun and the Moon was far too much for one single pony, as empowered as she was. Listening to their people, the Elements of Harmony chose a second noble mare. And so, Luna was empowered. Things would not be so easy, however. While they wanted to cooperate for the good of their ponies. Celestia and Luna had trouble finding a middle ground. Their endless disputes would bring Equestrian politics to a halt, and their bickering bring the kingdom to its knees, until one day, reality itself cracked, and Discord appeared. (because of the chaos their disputes was making) Celestia and Luna came to compromises, and fought Discord off together, bonding as sisters and soon making amends and treaties. Their peace lasted for a while, but Luna became jealous of her older sibling Nightmare Moon broke out.

Now, as the time approaches for Luna to return to Equestria, the elements are conversing again. Two princesses simply isn’t workable. Celestia and Luna are both good ponies, but are too distant in philosophies to rule cohesively. Equestria’s royalty needs a third member, an arbiter.

So, the elements begin their search for a worthy pony, and who would catch their eye if not a young lavender unicorn by the appropriate name of « Twilight »?

The rest, as they say, is history.