Chief of the Mearas.
Shadowfax was famed for his greatness as the horse of Gandalf, emissary of the Valar, in the War of the Ring.
In The Fellowship of the Ring:
Gandalf told the Council of Elrond of his gaining Shadowfax after the wizard's rescue from Saruman's tower by his flight on the great eagle, Gwaihir. The eagle landed him in Rohan when he requested a horse "surpassingly swift". There Gandalf found that the lies of Saruman had entered Rohan ahead of him so that Theoden, king of Rohan, would not heed his warnings of the treachery of Saruman and the coming of the Nine Ringwraiths. Instead the king told Gandalf to take a horse and be gone, not expecting the old man to take the finest horse they had.
Boromir said the Rohirrim, people of Rohan, would not even buy their lives with the horses they so treasured. These horses came from fields of the North, far from the Shadow, from the free days of old.
Gandalf described Shadowfax as a horse that might have been from the earliest time of the world. No man had ever ridden him before. By day his coat was like silver, by night like unseen shade. His footsteps were light, and not even the horses of the Nine were his match for speed or stamina.
Then Gandalf rode Shadowfax to track down Frodo who had the One Ring, as Gandalf learned in his time at Saruman's. His travel was made surpassingly fast over great distances. He went to Bree and on to Weathertop where he held off the Nine. He couldn not find Frodo in the wilderness and trusted to Aragorn to guide the hobbit while he drew the Nine away from them while riding to Rivendell for help. Thus only five of the Nine attacked the hobbits' camp. At Rivendell, Gandalf sent Shadowfax back to his master, King Theoden.
In The Two Towers:
Shadowfax returned to Rohan but was wild, suffering no man's touch, angering Theoden further with Gandalf.
In the chapter, "The White Rider", Shadowfax answered the whistle of Gandalf the White, also bringing with him the horses Eomer had earlier loaned to Aragorn's party: Arod (Legolas and Gimli's horse) and Hasufel (Aragorn's). Shadowfax was within range because Gandalf had bent his thoughts toward him the previous day. The other horses had run from their humans to greet their approaching chief, Shadowfax, leaving them unexpectedly without rides for a time.
Gandalf the White expected to take Shadowfax to battle, being then more than a counselor. By this time all the other Istari had left off their mission so that he was alone of their number. Gandalf allowed Shadowfax to choose their direction back to the King, as he was familiar with the terrain.
Due to the loss of the potential use of Shadowfax from wildness and for other reasons, King Theoden gave Gandalf and the three Fellowship members (Aragorn, Legolas, and Gimli) with him a cold welcome. But Gandalf helped the king see the truth: that Grima Wormtongue poisoned his mind and weakend his body with false counsel from his true master, Saruman. For the new life given him, Theoden showed his gratitude by making a permanent gift of Shadowfax to Gandalf, a gift that aided the wizard and many others thereby. Theoden said:
"There is none like to Shadowfax. In him one of the mighty steeds of old has returned. None such shall return again."
Eomer, who would be the next king of Rohan, saw the silver Shadowfax come to Gandalf's whistle, and said of the great horse:
"Were the breath of the West Wind to make a body visible, even so would it appear."
Theoden followed the advice of Gandalf, and Gandalf rode in support of him, as the cavalry of the Rohirrim went to stop Saruman's treachery and aid the land of Gondor which was about to come under siege by the armies of Mordor.
Gandalf and Shadowfax rode with the king at the head of the host looking for the king's men, led by Erkenbrand, who held back the forces of Saruman. News came that Erkenbrand's men fell back towards Helm's Deep.
Gandalf warned Theoden to take his people to Helm's Deep rather than the Fords of Isen, and left the host on his own errands. Shadowfax lent Gandalf the speed he had to have. He went apparently in several directions to locate Erkenbrand, then towards Isengard before nightfall to meet with Treebeard (and Merry and Pippin) for help against the orcs at Helm's Deep. Gamling had brought the regular folk of the area (Westfold) to Helm's Deep, but Theoden's fighters arrived before Erkenbrand's. Gandalf led Erkenbrand's infantry to break the siege of Helm's Deep, and himself on Shadowfax drove many of the orcs beneath the shadow of the Huorns, the dark ents, which had traveled to avenge the death of trees by orcs.
Then Gandalf rode through the Huorns, which parted for him, to Isengard with Theoden, Eomer and twenty of his men, Aragorn, Legolas, and Gimli. Wolves moved away from Gandalf as he rode, for fear of him and the shining horse. Shadowfax bore Gandalf to meet with Fangorn in Isengard.
Shadowfax bore both Gandalf and Merry on the trip back towards Helm's Deep. When the Nazgul flew across the river, Gandalf had all race as fast as they could, Shadowfax being the fastest even with his double load, then of Gandalf and Pippin who was recovering from looking into the Palantir. At that time, Pippin commented that Gandalf rode without saddle or bridle, elf-fashion as Gandalf called it. The wizard said that Shadowfax would wear no harness, carrying people or not as he pleased. Pippin also noted the unusual smoothness of the ride. But Gandalf and Pippin did not stop at Helm's Deep, heading straight on to Minas Tirith in Gondor.
In the Return of the King:
The book opened with Gandalf and Pippin riding Shadowfax. They stopped briefly at Meduseld, the nearly abandoned hall of King Theoden, where another Nazgul passed overhead. Gandalf saw the beacon fires in Gondor and wanted Shadowfax to speed up, but the horse had other ideas, slowing to greet three horses bearing messengers, then speeded mightily. They hid by day and raced along at night. When they entered Gondor, people saw Shadowfax and had hope that the Rohirrim were coming to aid, for they saw few other horses than those of the Steward's errand-riders. They had traveled one-hundred fifty leagues from Isengard. Shadowfax allowed himself to be taken to the stables when Gandalf spoke softly to him of it, for no horse was allowed in the Citadel.
Darkness fell with no dawn.
Nazgul, Black Riders of the air, sought to prevent Faramir and his men from re-entering Gondor. But Gandalf rode out on Shadowfax, the horse who had the courage to keep his mind before the Fear of the Nazgul. Gandalf spread light against their shadow, and stabbed at the Nazgul with a shaft of white light, sending the five away. Faramir carried precious news of the Ring's travel.
When Gandalf heard that Faramir and his forces were not only outnumbered ten times but up against the Witch-king as well as the other Nazgul, he went out on Shadowfax to give aid. Osgiliath was lost, and the forces headed back to Minas Tirith. Gandalf came in front with the wains of wounded and called for help from the city to aid the fighters' return. The regular enemy and the Nazgul came in a killing swoop at the rearguard led by Faramir, and would have overwhelmed them, but that Gandalf's Shadowfax outran Prince Imrahil's sortie from Minas Tirith and displayed his white fire. The Witch-king, leader of the Nazgul, not being there to challenge it, the others flew away for the time just as Faramir was hit by a Southron arrow/dart. The Morgul hosts, taken by surprise, broke away at the White Rider's coming, and were set upon by Faramir's outcompanies and the sortie.
The siege of Minas Tirith began.
Denethor went mad with despair from the news filtering to him past Sauron through the Palantir, made worse by the belief that his feverish son was dying and all his Stewardship. Pippin ran for Gandalf's help, but Gandalf was at the City Gate on Shadowfax. The Witch-king had come at last, possibly as strong as Gandalf's current form by Gandalf's words. The Witch-king's Words added their power to Grond, the battering ram, breaking down the Gate which had withstood Grond alone, in a blasting of lightning. But when he rode into the gate never before passed by an enemy, Gandalf waited on Shadowfax who "alone among the free horses of the earth endured the terror, unmoving, steadfast as a graven image in Rath Dinen." Gandalf knew well the prophecy of Glorfindel, that not by the hand of a man would this Nazgul die. But the pair halted the Witch-king's ride long enough that all heard the coming of the Rohirrim, with which the Witch-king had to turn away and deal. And there were two in that company who were not men. Gandalf started to ride out to the battle, but at Pippin's word had to stop and deal with the mad Steward Denethor to prevent Faramir's being burned alive, as no other could stop it, although Beregond delayed it. Shadowfax carried Gandalf and Pippin all the way to the Closed Door of the burial area, then open. Gandalf spoke to Shadowfax as to a man, asking him to return to the stables but come quickly if called, and the horse understood as would one of the ones of Numenor long ago.
Since Denethor was dead, Faramir his heir was mortally ill, and Prince Imrahil was in the fighting, only Gandalf was left to handle the City. He beheld with the sight that was given him what had happened with the Witch-king and understood how Sauron had reached into the city even to Denethor through the Palantir, and that this had prevented him from riding out on Shadowfax to battle. Then Gandalf was caught up in aiding the wounded, including Merry.
After Imrahil took over the lordship of the City for Gandalf, Gandalf was ready to ride Shadowfax in search of athelas to battle a new enemy, the Black Breath from the Nazgul, that was about to kill those in his care. Fortunately that delay was not required.
Gandalf, Aragorn, and others made plans and set out with a small army against the Black Gate of Mordor well away from Frodo's path, the direction known from Faramir's news. The army of less than six-thousand was bait to empty Mordor of as many of its dangers as possible to ease the passage of the Ringbearer, drawing the armies against themselves. Sauron was meant to believe Aragorn was the new Ringbearer setting out too early in his attack.
But once Sauron was defeated thanks to the terrible effort of the Ringbearer, and his dense, lightning-crowned shadow leaned over and stretched out its hand towards the small army, only to be blown away by a great wind, Shadowfax at last was not enough for the speed Gandalf needed. As the north wind became a gale sweeping away all the darkness, the wizard mounted the great eagle of Manwe, Gwaihir, taking his companion eagles, Landroval and Meneldor, and flew to collect Frodo and Sam.
The King returned, and all of Aragorn's long duty rewarded with a great marriage to Elrond's daughter, Arwen.
Shadowfax resumed his work with Gandalf, but in more pleasant circumstances. Their next long ride was in the peaceful faring forth of allies to return King Theoden's body to Rohan and for the announcement of the wedding of Eowyn and Faramir.
Then they began breaking up. Shadowfax bore Gandalf to Orthanc to check on Saruman, where he heard of yet another battle where ents destroyed orcs that surrounded Lothlorien but were unable to pass Galadriel and Celeborn. And also that Fangorn had let Saruman and Wormtongue go, since he deemed them defanged.
Gandalf along with Master Elrond, the Lord and Lady of Lothlorien, their company, and the hobbits continued riding and found Saruman along the way. Gandalf said he would still be able to do small, mean mischief.
The people of Lorien left them near Moria, and they rode on to Rivendell and Bilbo. Gandalf continued with only the hobbits as far as Bree to see Butterbur. There he heard of the troubles that had reached the lands of the hobbits, and set his well-trained hobbit fighters on their way to deal with it, proving that he had done his mission. Then Gandalf, finally alone, rode to see Tom Bombadil for a long visit.
But when Gandalf went to the Grey Havens to take ship back to Valinor, Shadowfax was not mentioned. He would most likely have traveled on Shadowfax to the Grey Havens, but what happened to his horse afterwards? Did he return to Tom Bombadil, Rohan, or ship to Valinor with Gandalf?
In The Letters of J.R.R. Tolkien:
JRR Tolkien addressed the question of what happened to Shadowfax in letter number 268. Here is a cut version of the letter giving parts relevant to the problem.
"I think Shadowfax certainly went with Gandalf, though this is not stated. I feel it is better not to state everything.... I should argue so: Shadowfax came of a special race being as it were an elvish equivalent of ordinary horses: his 'blood' came from 'West over Sea'. It would not be unfitting for him to 'go West'. Gandalf was not 'dying', or going by a special grace to the Western Land, before passing on 'beyond the circles of the world': he was going home, being plainly one of the 'immortals', and angelic emissary of the angelic governors (Valar) of the Earth. He would take or could take what he loved. Gandalf was last seen riding Shadowfax. He must have ridden to the Havens, and it is inconcievable that he would [have] ridden any beast but Shadowfax, so Shadowfax must have been there."
Fellowship: "The Council of Elrond".
Two Towers: "The White Rider", "The King of the Golden Hall", "Helm's Deep", "The Road to Isengard", "The Palantir"
Return of the King: "Minas Tirith", "The Siege of Gondor", "The Pyre of Denethor", "The Black Gate Opens", "Many Partings", "Homeward Bound", "The Grey Havens":
# 268 The Letters of JRR Tolkien
Images of Shadowfax:
image - "Gandalf and Pippin", June 1998. Gandalf and Pippin ride Shadowfax under the moonlight with a storm behind them, racing to Minas Tirith ahead of the army of the Rohirrim. Artist Luca Michelucci . From Rolozo's Tolkien page.
image : "Shadowfax". Gandalf riding Shadowfax through mist. Artist Michelucci, 1998 Tolkien calendar. Rolozo page
Image: "Visiting Shadowfax". Artist: Anke-Katrin Eissman. Rolozo Tolkien site