In the magical land of Equestria, two alicorn sisters raise the Sun and the Moon every day to give the world a night and day cycle, basic for the survival of life. There are also two alicorns who rule over two very core aspects of life and harmony itself. Those are Love and Friendship. While not as ancient as the two sisters, all princesses are essential to the world's balance.
This balance is also sustained by the hard-working ponies, who make the seasons change and manage the weather to fit their needs, so everypony has enough food and water, and every animal is also attended to assure that any of them are caught in the changes of seasons.

But what about the planet itself? What about Mother Nature?
This is the question that Gaia asks when she arrives to this foreign world. After so much work her son persuaded her to take a long vacation in Equestria, saying that she needed one and assuring her that he would be in charge now, and she must take a break. But what can a several billion-year old alicorn possibly do to pass her time in this land?

(SimEarth/MLP Crossover)
(Timelife of Gaia's life 'till the start of the story)

"When you were brought back, you signed an unspoken agreement, took an invisible oath; to protect not only the Traveler, humanity, and the last city, but to also purge the darkness with your light. No matter how far you must travel. No matter how many stars you must blur by, the darkness shall not escape the wrath of your light."

A phrase which a certain guardian holds very close to their heart. But is that even possible when their memories and humanity were taken away in an obscure accident? So now they must rediscover who they are with the scraps left behind, and rediscover why they arrived here, before it's too late for them, and the residents of Equestria.

Crossover with Destiny.

Luna does her best to catch up on the events of the past thousand years and ends up reading even more than Twilight.

One day, an ancient tome slips into her pile of reading material and she reads it eagerly. What she finds there may very well be the discovery of the millennium: The location of the last refuge of the humans.

With the help of Twilight and her friends, the Lunar Princess sets out on an archaeological expedition to find the very last human on the planet of Equus.

And they found one, but why is he forged in a full body armor and claims to not being a human... Well he's not one anymore.

After his latest plans were foiled by that pesky blue rodent, Dr. Robotnik finds himself in the land of Equestria. His quest for world domination in this strange planet begins. With no speedy hedgehog around is there anything that can stop the mad genius from taking over?

A My little pony and Sonic the hedgehog crossover fic, featuring Dr. Robotnik (Eggman),