Equestria was a place that was founded by the rulers of each tribe, earth pony, pegasus, and unicorn. It was said that because the rulers of each tribe bonded together in friendship, that's what ridded their world of Wendigos. So a holiday, called Hearth's Warming Eve was established to remember how the founders bonded together in friendship and got rid of the Wendigos to keep Equestria safe.
Little did the Founders of Equestria know that before Equestria was founded, the land that became Equestria was already being protected by a group of powerful ponies. This group of ponies called themselves the Guardians. They were able to protect the land by what they called Gates. These gates are portals that detect problems with unstable magic in the land and when a guardian opens up a gate, they send that unstable magic to a dimension to keep the land safe from harm.
Originally, there were five guardians, one at each gate. The gates were in the North, South, West, and East, with a Center Gate that sat on top of the land's connection of ley lines, or magic currents. As time went on, one of the guardians, Sapphire Rose, became stronger than all of the guardians combined.
As such, the other guardians decided that Sapphire Rose would be the one to be the leader over all of them. At that time, she was the guardian of the Center Gate. With that being said, Sapphire also decided that she not only needed another guardian to take her place as the Center Gate Guardian, but she also needed somepony to help her in making sure that the guardians were kept stable and sound in mind in order to do their jobs.
The Guardians were the ones that truly stopped the Wendigos from overtaking the land. The Founders of Equestria had no idea this happened when they bonded over friendship.
Now that Sapphire was the leader of the Guardians, she stepped forward and talked to the founders at length about how to stop similar threats like the Wendigos.
However, during their discussions, Sapphire knew that the Founders were the ones who caused the Wendigos to happen in the first place. Because of them, they almost turned the land she protected into a wasteland where no creature, not even the Guardians would be able to live. Seeing that she was trying to make peace with the Founders so she and the other guardians wouldn't have to drive them out of the land, Sapphire chose not to mention that.
Wendigos are the hardest type of unstable magical creatures to keep locked inside the unstabilized dimension.
Sapphire and the Founders of Equestria agreed to work together, to keep Equestria safe.
Then the Elements of Harmony were created by a wizard named Star Swirl the Bearded. Sapphire saw this as a way to make the gates stronger and add more protection for Equestria. So, she talked to him about this and he agreed with her, as the Elements themselves were grown inside a leyline current.
Now, every two hundred years, because that's how long these chosen ponies live, new Guardians are chosen to connect to the gates. The Elements of Harmony also get paired up with a Guardian every time the year of choosing the Guardians comes around.
The Gates of Equestria isn't common knowledge to everyone. Only those that picked represent their nation and city within Equestria know of the gates. Sapphire is the one that picks the city's representatives too.
Sapphire's one goal in her life that keeps her going more than anything else, is to protect Equestria. The Guardians and the Elements of Harmony feel the same way.