Actually, the LOTR water-horses were totally unconnected, I just thought about that scene in the movie when I was trying to explain how the foam beasts seem to form from foam and solidify into real bodies. The horses were inspired by the Celtic Kelpies, Aughiskies (
Each Uisge), Doonies, and other water horses and the Froth Hounds's appearance was inspired by the Celtic 
Cu Sidh. As I said toward the end, the sea-dwelling races have tales of seeing apparitions of the aphrotheres in the deep oceans; but like I said in the beginning, land-dwelling races by the sea have fewer tales of them than those by rivers. 
And I'm VERY glad that you like my water-elementals, these spirits were a little harder to work on than the earth or fire elementals 
