Hi fella, I'm going to answer your questions. As the author wrote in the description of all these "Zodiac ponies" they're based on Saint Seiya, a japanese anime based on greek mythology, where the 88 greek constelations are represented as armours and 88 young men and women are chosen to wear them in order to protect the world and the goddess Athenea.

There are three types of armours, bronze, the main characters in the show wear these armours, silver and gold; the golden armours are the strongest and are represented by the twelve zodiac signs.

In the show, the bronze knights have to fight against the twelve golden knights to save Athenea's life, going from Aries to Pisces. When they get to Capricorn, they find out that he's the most loyal knight to Athenea, so, she granted his loyalty giving him the most powerful sword ever known, Excalibur.

All that is just part of the show and that's how they are related.

I hope I could answer your questions, and I'm sorry for the long explanation, but I don't know if you know about Saint Seiya's show.