This is the work off a genius and that my friend is you. I mean first off,The vision. I mean it's amazing,I love how you played with the colours in the background that leaps from light-blue from dark-purple. Also the thin colours around her body are pretty aswell,the same goes for the forms and lines and shadows.
Than Originality. An amazing design for a sea-pony,what I kinda expected from you,because you design so many beautiful fantasy stuff.
Now the technique. Try to work on the left hoof. It almost look like her hoof is shorter than her right hoof. Still the other lines were amazingly perfect and for the rest :
Than the impact. When I first looked at this,I felt like I was scuba-diving deep under the sea and than discovering this beautiful animal. And believe me,I don't have that experience much,so it gave an great impact on me.