Princess Fillies are best recognised by their lack of distinct features: They have no wings, no horns, no capes, and normal ears. They also all have titles (e.g. queen, duke) as part of their names, although some other Filly types include such names.
Story from the MyFilly site
Somewhere across the deep blue ocean and stretching as far as the eye can see, there is a small and beautiful island with long white beaches, tall green trees and nature in all the bright colours that you can imagine. This island is the home of the royal Filly family - the magnificent Filly Kingdom.
The Filly live in peace and harmony, enjoying their lives and the beautiful gardens and forests surrounding them. The Filly families live in amazing palaces with magical colours, and all wear golden crowns as they part [sic] of the royal family.
Every year Emperor Karus invites his relatives from the other palaces in the Filly Kingdom to an incredible party, where all the Filly meet and dance from sunset to sunrise under the shining stars.
A special guest is attending - the adorable and mysterious Princess Crystal, with the magical crystal in her golden crown, speading [sic] fantastic gifts to the other Filly. But before midnight she disappears, and the tale says that she returns to a magical land far away.