Eras

Equestria's history is sorted into many different Eras. Each era lasts for a thousand years, which is assumed to be the result of some sort of magical significance of thousand-year time spans.

Pre-Classical Era
The three Pony tribes, the Earth Ponies, Unicorns and Pegasi lived apart from each other, and only interacted with each other through trade for the sake of survival (the Earth Ponies ran most farms which supplied the tribes with food from their farms, the Pegasi controlled the weather, and the Unicorns used spells they had been granted by Sola and Lycan to control the Sun and Moon).

Classical Era
After Hearth's Warming Eve, the three tribes united to create modern Equestria. The princesses Celestia and Luna were born, and taught by Star Swirl the Bearded the spells which they would use to control the Sun and Moon themselves.

Chaotic Era
Discord and the Draconequus began a war against Equestria, until Sola and Lycan themselves had to interfere. Discord managed to kill Lycan, and absorbed his power (the remnants of his soul scattered across Equestria to create the Lycanthropes). Sola created the Elements of Harmony, which Celestia and Luna used to seal Discord in stone and defeat the Draconequus.

Royal Era
Celestia and Luna ruled over Equestria in peace for a thousand years.

Solar Era
The Crystal Empire was taken over by King Sombra, who was defeated by Celestia and Luna. He managed to curse the city to disappear for a thousand years before his defeat, however.

Luna became jealous of Celestia and turned into Nightmare Moon. Using the Elements of Harmony, Celestia sealed Nightmare Moon in the moon for a thousand years. She created the Everfree, which grew to consume the Castle of the Royal Pony Sisters. Canterlot became the new capital of Equestria, and the Summer Sun Celebration and Nightmare Night holidays were created.

Modern Era
Nightmare Moon returned, but was restored to being Luna by the Elements of Harmony, wielded by Twilight Sparkle, Rarity, Applejack, Pinkie Pie, Rainbow Dash and Fluttershy. Luna returned to her throne alongside Celestia.