Shadowfax was the horse featured in the trilogy of the rings. He was the Lord of all horses. Shadowfax was a descendant of Felarof from the line of Mearas. The Mearas were considred the greatest horses of Middle-earth.
Shadowfax could understand human speech. He could when needed run faster than even the wind. He belonged initially to the House of Eorl, the Lord of Rohan. However, he turned out to be too wild to be tamed by the Rohirrim. His coat was described as silvery-grey. It was said that Shadowfax could not be seen at night.
Shadowfax was given to Gandalf the Wizzard. Gandalf rode Shadowfax during the seige of Gondor. The Wizzard also rode Shadowfax during the Final Battle at the Black Gate. Gandalf wrote this great and valiant horse home after the ring was destroyed in the fires of Mordor.
Note: I was so impressed with Shadowfax after reading the Trilogy and obtaining the DVD's of the series that I named my boat Shadowfax.