The Mermaid Fillies are Fillys with fish tails for their lower body. They have two home lands for themselves, Aquatica and Coralia, which are worlds underneath the Filly Oceans. They are often musically inclined, and loves to live the life underwater without much fewer things to do than in the above worlds. In fact, the Mermaid Fillies have even figured out things like drinking squash and painting paintings underwater.
All Mermaid Fillies have tails, in most cases fish tails, below their waist. In general, most Mermaid Fillies' tails look different, with a few standouts such as Fauna's more fish-like and less mermaid-like tail. Two Fillys that are closely related, although they're not required to be parent/children or two spouses, generally have tails with more or less identical colors, although two Fillys' tail colors are never the same as that of another two Fillys of the same land. Two from Aquatica and two from Coralia may still have very similar tail colors.
The tails can be used to swim, logically enough. They exhibit a large amount of control of their tails, such as being able to sit upright with the tail, and have even been surfing while standing on their tails like a snake.
They can also breathe underwater as if it was normal air. There are no visible gills on the mermaids, and they are not known to be required to swim up to the surface on occasions to stock up on air in the lungs. They can also breathe above the water surface without noticeable problems, for things such as sunbathing on a rock or to hear about news from the above world.
Similar to many other Filly types, golden crowns with crystals are used on occasions, and while Aquatica abandoned it in favor of pink crowns with seashell pearls, Coralia went the other way by replacing their pink crowns with golden ones.
A rule of thumb is that the the female Mermaids wear necklaces, while the male Mermaids wear bracelets.