So, here’s one of my pony OCs. She is an incredibly Mary Sue-ish OC, yet she was really one of my first pony OCs and I like her anyways.
 
Gaia was at one point in time, an alicorn, alongside the Celestial sisters. In my headcannon, at one point in time there were many alicorns. Gaia was the alicorn who was responsible for controlling the systems of weather that ran the Earth, her most important job being control over the water cycle. Gaia’s cutie mark is an image of the Earth itself.  
When Celestia and Luna overthrew Discord, they offered Gaia a place as a Princess. The alicorn saw the danger in the government Celestia set up, where royalty had all the power. Gaia refused. Celestia would have been fine…except Gaia was stupid. Gaia was very loudmouthed about her opinions on Celestia’s rule, calling the sisters “the next Discord”. She preached that with the power they now had upon them, the sisters would eventually end up no different from Equestria’s last ruler. Celestia saw the danger in Gaia spreading this information, but did not do anything about it.  
It was when Gaia began to rally ponies to rebel against the Princesses that Celestia decided she must take action. Gaia invaded Canterlot, and began to wreck havoc on the city with a group of rebels. Before long, the residents of Canterlot got involved and there was a full on riot. Celestia was able to stop it on time.  
In Punishment, she cast a spell on Gaia. She took away the alicorn’s horn and wings, and replaced them with fins. She turned Gaia into the first ever Kelpie Pony. Some of Gaia’s closest rebel allies also got this punishment.  
My MLP-ifed version of Kelpies differs a bit from the monster in folklore. Kelpie Ponies are more than just water ponies, they are monsters. They will often kill ponies (ether for food, or just out of instinct) along with other wild animals. When hunting or angered, they will have very little conscious control over themselves and will attack just about anything that comes near them. Some Kelpies (including Gaia) try to control these savage instincts, yet occasionally they will slip up. Gaia is having one of those “savage” moments in this picture.