Welcome, as ever, dear readers.
As I was thinking and writing about Icelandic horses, a couple of postings ago, Gandalf popped into my mind—rather like William Morris of that posting—bearded and on horseback,

(a caricature by his friend, Edward Burne-Jones)

(the Hildebrandts?)
although a bit more imposing.
In this illustration, as in many, he’s mounted on Shadowfax, whose name appears to be based upon two Old English words, scead, “shade/shadow” and feax, “hair/mane”, suggesting “Shadowmane” in English, just as Theoden’s fatal horse is “Snowmane” (combining Old English snaw/snawa with feax?).

(by Joon Tulikoura)
Although his mane may be shadowy, Shadowfax appears to be white, with a silvery sheen, as Gandalf describes him:
“The horses of the Nine cannot vie with him; tireless, swift as the flowing wind. Shadowfax they called him. By day his coat glistens like silver; and by night it is like a shade, and he passes unseen.” (The Fellowship of the Ring, Book Two, Chapter 2, “The Council of Elrond”)
These are not the only white horses in The Lord of the Rings. When Glorfindel appears, his horse, Asfaloth, is also white and, astride it, on the far side of the Ford of Bruinen, as the river begins to rise, Frodo thinks he sees:
“…a plumed cavalry of waves. White flames seemed to Frodo to flicker on their crests, and he half fancied that he saw amid the water white riders upon white horses with frothing manes.” (The Fellowship of the Ring, Book One, Chapter 12, “Flight to the Ford”)

(Ted Nasmith)
(We later find out that, although Elrond commanded the waters of the river to rise, Gandalf “…added a few touches of my own: you may not have noticed, but some of the waves took the form of great white horses with shining white riders.” The Fellowship of the Ring, Book Two, Chapter 1, “Many Meetings”)
I’ve always imagined that Shadowfax is white because
1. he’s the descendant of the horse ridden by Eorl the Young, as we see him in the tapestry in Meduseld—
“Many woven cloths were hung upon the walls, and over the wide spaces marched figures of ancient legend, some dim with years, some darkling in the shade. But upon one form the sunlight fell: a young man upon a white horse. He was blowing a great horn, and his yellow hair was flying in the wind. The horse’s head was lifted, and its nostrils were wide and red as it neighed, smelling battle afar. Foaming water, green and white, rushed and curled about its knees.
‘Behold Eorl the Young!’ said Aragorn. ‘Thus he rode out of the North to the Battle of the Field of Celebrant.’ “ (The Two Towers, Book Three, Chapter 6, “The King of the Golden Hall”)

(Alan Lee)
2. he now matches Gandalf, who has become Gandalf the White,

(the Hildebrandts)
as Aragorn hails him:
“Then suddenly [Gandalf] threw back his grey cloak, and cast aside his hat, and leaped to horseback. He wore no helm nor mail. His snowy hair flew free in the wind, his white robes shone dazzling in the sun.
‘Behold the White Rider!’ cried Aragorn, and all took up the words.” (The Two Towers, Book Three, Chapter 5, “The White Rider”)
3. the whiteness of the two is a strong contrast to the blackness of the chief of the Nazgul when Gandalf encounters him at the gate of Minas Tirith—
“In rode the Lord of the Nazgul. A great black shape against the fires beyond he loomed up, grown to a vast menace of despair. In rode the Lord of the Nazgul, under the archway which no enemy ever yet had passed, and all fled before his face.
All save one. There waiting, silent and still in the space before the Gate, sat Gandalf upon Shadowfax. Shadowfax, who alone among the free horses of the earth endured the terror, unmoving, steadfast as a graven image in Rath Dinen.” (The Return of the King, Book Five, Chapter 4, “The Siege of Gondor”)

Of course, important people on white horses appear all the time in history.
Here, for instance, is Napoleon, painted on his white Arabian horse, “Marengo”—

(Baron Gros)
and George Washington, on an unnamed horse.

(John Faed—for an interesting article on Washington’s horses, see: https://www.mountvernon.org/george-washington/farming/the-animals-on-george-washingtons-farm/horses/ )
And how could we forget the Lone Ranger’s horse, “Silver”, both in his 1950s form

and his 2013 reincarnation?

But there’s a more sinister white horse which might also have been in the back of Tolkien’s mind as a long-time Bible reader:
“6 And I saw when the Lamb opened one of the seals, and I heard, as it were the noise of thunder, one of the four beasts saying, Come and see.
2 And I saw, and behold a white horse: and he that sat on him had a bow; and a crown was given unto him: and he went forth conquering, and to conquer.” (The Book of Revelation, Chapter 6, Verses 1-2)

(V.M.Vasnetsov)
There has been much discussion for centuries as to what this figure signifies, but, at his confrontation with the Nazgul, Gandalf says:
“ ‘You cannot enter here…Go back to the abyss prepared for you! Go back! Fall into the nothingness that awaits you and your master. Go!’ “(The Return of the King, Book Five, Chapter 4, “The Siege of Gondor”)
The Lord of the Nazgul laughs at this, but, soon after, over the body of Theoden and his fallen white horse, Gandalf’s words come true and he who thinks to conquer is himself conquered.

(This is from an older site called “Shadowcore”, where the artist, Craig J. Spearing, presents a very informative picture of himself at work: https://shadowcoreillustration.blogspot.com/2011/10/eowyn-nazgul-process.html )
As ever, thanks for reading.
Stay well,
Plan to bluff a Nazgul,
And remember that there’s always
MTCIDC
O