Actually, Cassie looks different enough to be from a different but related species. Personally I like the idea that Sirens are the largest species in a family of aquatic dragons resembling Ponies. Said family also includes Mermares, Seaponies and Kelpies. They are distantly related to Kirins and Longmas, other dragons resembling Ponies.A bestiary entry for Sirens could probably read like that:

"The Creator, if she exists, has an inordinate fondness of Ponies and things that look like them." - famous Griffon Naturalist Hawk, often(mainly by Ponies) wrongly attributed to Starswirl the Bearded.

Thalattodracon, largest of the Thalattodracoidea, commonly known as Sirens. Admired for their beauty, hunted for their magical properties, feared for their power. Although they resemble ponies, they are actually a mostly herbivorous species of marine dragon, descended from a land-dwelling ancestor. Individuals vary greatly in color and personality, but they are social creatures and tend to form schools of three or more Sirens. They are generally considered docile, but can be quite aggressive if necessary, using their powerful voices, which are often said to have a hypnotic effect. The gem on their chest is called a draconite or dragon stone. It is considered precious for both, jewelry and magic, making Sirens a target for those who are after it. Together with low birth rates this lead to their near extinction in the past.



