The Éothéod was the name the former Northmen took after they were divided during the Battle of the Plains. Those who gathered to Marhwini in the Vales of Anduin took this name meaning ‘Horse-people’. After they outgrew their lands there, they followed their leader Frumgar into the north to the sources of Anduin. After the defeat of the Witch-king the Éothéod then moved into the lands of the former realm of Angmar. There they dwelt until they moved to Rohan after the Battle of the Field of Celebrant.

To read more about the Éothéod, see the link above to the Middle-earth article called ‘A History of The Rohirrim’.
Encyclopedia entry originally written by Nienna-of-the-Valar