Shadowfax: King of the Mearas by paulab
(Legolas speaking) ... 'See how they run! There is Hasufel, And there is my friend Arod beside him. But there is another that strides ahead: A very great horse. I have not seen his like before.' 'Nor will you again,' said Gandalf. 'That is Shadowfax. He is the chief of the Mearas, lords of horses, and not even Theoden, King of Rohan, has ever looked on a better. Does he not shine like silver, and run as smoothly as a swift stream? He has come for me: the horse of the White Rider. We are going to battle together.'
-J.R.R. Tolkien: The Lord of the Rings: Book 3, chapter 5, 'The White Rider'