Shadowfax is the Lord of the Mearas, an exceptionally swift and intelligent breed of horses from Rohan. He’s a friend of Gandalf and carries him into battle. While still under the influence of Wormtongue, Théoden is angry with Gandalf for taking Shadowfax, but he later gifts Shadowfax to Gandalf in thanks for exposing Wormtongue as a spy for Saruman.