The Legends

Holy crud, I screwed up on Luna.

But I have HEADCANNNONZ!

Alicorns, the ascendent form of the pony races. They are powerful, mostly because their main purpose is to protect the ponies they rule. In the pony world, political power is measured in physical power. Only Alicorns can wield the Elements of Harmony to their fullest potential, and they can dish out and take more damage than any ordinary creature. Alicorn magic is a mix of pegusus, earth pony, and unicorn magic, all taken to their highest potential. They fly better, run better, and cast magic better than any other pony. Alicorns live about 5,000 years on average (except for the regents of the sun and moon, who live twice as long), compared to the normal 130 years or so for normal ponies, and they need less sleep and food.

Note: "Artificial" Alicorns, like Twilight, have almost no difference between them and "Natural" Alicorns, like Celestia and Luna, in my headcannon.

Other Note: In my headcannon, Cadence is a natural Alicorn, it just makes more sense to me.

So, Celestia, her Highness Queen Celestial Light, Regent of the Sun and Guardian of the Light. She wields the Elements of Patience, Kindness, and Generosity, and she kicks rump. She can really dish out a lot of power and she specializes in hitting hard and fast, but she can take as much damage as she can dish out. Almost no one can stand up to the mighty power of the sun, except her sister, Luna. In my headcannon, Celestia suffered huge emotional trauma from sending her sister to the moon. Celestia isn't crazy yet, but the strain of using all the Elements by herself, sending her sister and only friend to the moon, and then spending nearly a thousand years alone has brought her pretty close to the breaking point. So she seems distant and regal to most ponies because she is trying to hide her broken heart. However, over the course of the story she lightens up a lot, and forms a sort of family with the mane seven and Cadence and Shining Armor. Because she has lost so much, she is very protective of her “Family” and will defend them at all costs, especially Luna and Twilight. Celestia is legendary for being almost impossible to take down, not because she dodges well, but because she can take almost any hit and keep fighting. She is about 2,500 years old, but is as mature as a 30 year old.

Luna, her Highness Queen Lunar Fire, Regent of the Moon and Guardian of the Light. She wields the elements of Loyalty, Honesty, and Laughter. Her fighting style is more centered around pinpoint hits, and she is one of the fastest and sneakiest ponies alive, so she is nigh impossible to hit (she can't take much damage though). In my headcannon Luna is dealing with massive guilt over becoming a monster and hurting her sister who raised her. Almost everyone expects her to turn into Nightmare Moon at the drop of a hat and many nations don't believe that an Alicorn could be hiding this long, and they tend to see Luna as some sort of power play by Celestia. But Luna has her sister, the mane seven and Cadence and Shining and she is happy. Most of Luna's time is taken up with healing her sister's emotional scars, and protecting the dreams of ponies from the various beasts who would do them harm. She also runs the Lunar Guard, which is an army made of former black magic practices. Luna believes highly in second chances, and also in kicking the butt of evil doers. Luna is about 2,000 years old, or about 25 in normal years

The Alicorn sisters are very important in my headcannon, because unlike in the show where they are basically has beens, in my headcannon they still wield the elements (more on that later). Luna and Celestia are Equestria's last and best line of defense, and it isn't unknown for them to defeat entire armies. On their off time, they go out for ice cream, bug all of Ponyville and argue about tea, coffee, fudge, and cake.