Shadowfax Mearas Sacrifice

Shadowfax was a descendant of Felaróf, and a chieftain of the race of long-lived Mearas, the greatest horses of Middle-earth. Shadowfax was capable of comprehending human speech and was said to run faster than the wind. Originally belonging to Théoden, King of Rohan, Shadowfax was too wild to be tamed by the Rohirrim. Eventually, Théoden gave him to the wizard Gandalf the White
Shadowfax's coat is described as silvery-grey in daylight, and hardly visible at night.
J.R.R. Tolkien's inspiration to give the horse the name Shadowfax (Shadow Mane) was presumably the most extensive source for Norse mythology, "The Prose Edda" (e.g. Hrimfaxi (Frost Mane) and Skinfaxi (Shining Mane)).