In Tolkien’s legendarium, Gwaihir is the great eagle who rescues Gandalf from Orthanc.
Gwaihir the Windlord is the greatest of the Eagles in the late Third Age. In The Fellowship of the Ring, Gandalf sends a message to Radagast, who arranges for Gwaihir to find him after Saruman imprisons him on the summit of Orthanc. Gwaihir carries Gandalf away to safety.
Gwaihir later rescues Gandalf again after the wizard’s battle with the Balrog on the peak of Zirakzigil. Near the end of The Lord of the Rings, he also leads Eagles to the field of battle at the Black Gate and helps rescue Frodo and Sam from Mount Doom.
Gwaihir is sometimes confused with other named Eagles. Landroval is his brother, while Thorondor is the great Eagle-lord of the First Age. Gwaihir therefore belongs primarily to the stories of the Third Age, not the earlier wars described in The Silmarillion.