It turns out that the white dragon was born like this, just an old horse.
After 1.3, thousands of horses of various races were introduced (as you can see now). Old color-changing horses can be replaced with new ones for free, so NPC has a dialogue to recycle white dragons.
The same is true of other races.
Now if anyone has a white dragon, it's just what he bought before and hasn't changed it.
However, when CWOW opened the internal test, it was already 1.3.
So it is impossible for anyone to have such a discolored horse.