'There is more than one horse coming,' said Aragorn.
'Certainly,' said Gandalf. 'We are too great a burden for one.'
'There are three,' said Legolas, gazing out over the plain. '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 Théoden, 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.'
Even as the old wizard spoke, the great horse came striding up the slope towards them; his coat was glistening and his mane flowing in the wind of his speed. The two others followed, now far behind. As soon as Shadowfax saw Gandalf, he checked his pace and whinnied loudly; then trotting gently forward he stooped his proud head and nuzzled his great nostrils against the old man's neck.
Gandalf caressed him. 'It is a long way from Rivendell, my friend,' he said; 'but you are wise and swift and come at need. Far let us ride now together, and part not in this world again!'The Lord of the Rings: The Two Towers
, "The White Rider"( slightly different pic )