An Old Guardspony's Last Duty

An Old Guardspony's Last Duty is a short story by BuffaloBrony. It is related to And Pitiless As the Sun by the same author.

Summary
Summer Oak is a young colt attending Princess Celestia's School for Gifted Unicorns. When a lab accident causes a fire, Summer Oak instinctively casts a shield spell, leads his fellow students out of the building, and then goes back in and rescues some more, earning him his cutie mark.

Years later, Summer Oak has joined the Guard. The day before his graduation, he is summoned to a bar in Canterlot. There, he finds not only his sergeant, but a number of other guardsponies who have assembled for a retirement party for Starlight Dancer, the Knight Captain of the Royal Guard.

Said Knight Captain, Starlight Dancer, takes Summer Oak aside and offers him a position in the Royal Guard, explaining why it takes a special kind of pony to join the subdivision instead of the more conventional and general-purpose Equestrian Guard. Reminded of how naturally leadership skills kicked in when he was a colt, Summer Oak accepts.

After the new recruit as well as the rest of the party has left, Starlight Dancer turns to a barmare. She drops her magical disguise and is revealed as Celestia. She has heard he entire conversation, and she approves of Starlight's choice; Summer Oak reminds her of Starlight when he was younger.

BuffaloBrony's Royal Guard
The Royal Guard is a subdivision of the Equestrian Guard nominally tasked with the personal protection of the Princesses. In this story, the reason they all look the same is because of spells woven into the fabric of their armor. All guardsponies of a certain type look identical to an outside observer for the sake of civilians (to be easier to recognize as a figure of authority), to confuse would-be attackers, and for the sake of the Princesses (to be less easy to recognize so their inevitable death isn't as painful to the immortal alicorns).

Contary to what Summer Oak initially believes, the Royal Guard is only superficially a protection detail for the Princesses. As Starlight wryly observes, most of the big, physical threats tend to be handled by the Elements of Harmony. Instead, the Royal Guard's mission is twofold: take charge in situations where no one else is keeping a clear head and keep civilians from panicking; and keeping the Princesses from going insane by having an open ear and doing anything they desire.

This interpretation of the Royal Guard is also present in And Pitiless As the Sun, and the author has announced a sequel.