Needs Horse Ranch, of course. I took one of the mermaid tails from Island Living and slapped it on a horse butt. More specifically, I gave it the weight paint of a horse tail of course, but the textures, even the specular, are all from the original, made to look as exact as possible. No extra swatches. I made 3 versions, one is 1:1 the exact same as the original mesh, the second one, seen in the ingame screenshots above, is made longer, and the third one I manipulated the mesh a little bit to have some more depth to the tailfin. The animation isn't perfect, the mesh deforms a bit sometimes, but it looks fine. None of the other CC on the horse is mine. I can't tell you what it is either as I have basically every bit of horse CC I have seen. :)
I attempted making body fins to go with this, but they kept breaking ingame so not now. Maybe some day I'll try them again.