Shadowfax is the fastest horse of Middle-earth and is a decendant of the mighty Mearas. He was tamed by Gandalf who then "barrowed" him from King Theoden. After Gandalf cured King Theoden, Shadowfax was given to Gandalf. Shadowfax's legendary speed was of vital importance for Gandalf's missions and it allowed him to make many just in time appearances. Shadawfax was always riden bare back and it is believed that he left Middle-earth together with his master Gandalf.
"Shall we continue to our next creature: Treebeard?"