That is Shadowfax. He is cheif of the Maeras, lords of all 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?
“…a flash of white and silver coming from the North, like a small star down on the dusky fields…”
The Return of the King, Book V, Chapter IV: The Siege of Gondor
Pippin found that Shadowfax had been well housed and tended. For in the sixth circle, outside the walls of the citadel, there were some fair stables where a few swift horses were kept, hard by the lodgings of the errand-riders of the Lord: messengers always ready to go at the urgent command of Denethor or his chief captains. But now all the horses and the riders were out and away.
Shadowfax whinnied as Pippin entered the stable and turned his head. ‘Good morning!’ said Pippin. ‘Gandalf will come as soon as he may. He is busy, but he sends greetings, and I am to see that all is well with you; and you resting, I hope, after your long labours.’
Shadowfax tossed his head and stamped. But he allowed Beregond to handle his head gently and stroke his great flanks.
- The Lord of the Rings: The Return of the King, by J. R. R. Tolkien