Star-dance Seafoam doesn't know what she is, or anything Mère Gothel didn't teach her... except for the rare visions she sees when she swims that Mère Gothel tells her are only her imagination.
(This is not a crossover & the only anthro is the main character who is a siren though more human than hippokampos)

My name is Yin-Yang. It has been about a month since I moved to Ponyville; in that short time, I have become recognized as the Golden Oaks’ librarian. I've been told that Twilight Sparkle, will be arriving on schedule to check on the preparations for the upcoming Summer Sun Celebration.