Nazgul horses

[Gordis]

Hi, all!
I have made a curious observation. I hope it was not discussed before.

After the nazgul were drowned at the Bruinen Ford only the bodies of 8 horses were found:

Three of the black horses had been found at once drowned in the flooded Ford. On the rocks of the rapids below it searchers discovered the bodies of five more, and also a long black cloak, slashed and tattered. Of the Black Riders no other trace was to be seen, and nowhere was their presence to be felt. It seemed that they had vanished from the North.
'Eight out of the Nine are accounted for at least,' said Gandalf. 'It is rash to be too sure, yet I think that we may hope now that the Ringwraiths were scattered, and have been obliged to return as best they could to their Master in Mordor, empty and shapeless.

So, as you can see, one horse may have remained alive.

Isn't it curious that later only one of the nazgul (the Witch-King) was seen to ride a horse at the siege of Minas Tirith? The rest were constantly on Fell Beasts.

I believe that horse was the same animal that survived the Flood.

So it looks like:
1. One of the Black Riders (most likely the WK) was able to get out of the river without "loosing his shape" (whatever that means) and save a horse and ride to Mordor ahead of the others. Later he probably collected the rest of his Riders on the Fell Beast (remember the shape that flew over the Fellowship near Caradhras)
2. The nazgul had only one trained horse each
3. The training took a long time, not just a single spell, but probably a lot of petting and carrot-feeding

What do you think on it?