The Eagle is an unnamed character in Spirit: Stallion of the Cimarron. The eagle is affiliated with the Herd of the Spirit, particularly the Spirit himself. The eagle is shown to be very attached to the Spirit, and he searches tirelessly for him when he is taken from the flock.
Without him, we would never understand how fast and wild the Spirit really is. He is an important character because it shows how before the white invaders came and destroyed all the land that so obviously belonged to them, the animals lived together in peace.
He seems to care about Spirit, but he was obviously hurt when he saw the stallion in a new place with horses. He must have felt Spirit left him and his flock behind, so he turned. He was very happy when Spirit returned.
The Eagle is Spirit's friend and is depicted as a symbol of Spirit's freedom. When Spirit grew older, finally transforming into the over-stallion of his herd, Spirit celebrates by running with the eagle flying overhead, joyously bleating and whinnying, showing the two are close friends.