During the Third Age, first mention of the the Éothéod is when they migrated under their king Frumgar to the confined area between the rivers Langwell and Greylin, sources of the Great River Anduin, near where the Ered Mithrin met the Misty Mountains. They went that way after the fall of Angmar, away from the ravages of the Easterlings and Orcs.
Some time later their king Fram, son of Frumgar, slew the dragon Scatha. The Éothéod capital was named Framsburg in his honour. Fram's son Léod was killed trying to tame the horse Felaróf, first of the Mearas of Rohan. His son Eorl the Young tamed the horse, taking it into service as compensation for his father's life.
During the rule of the ruling Steward of Gondor Cirion, Gondor faced an attack by the evil Balchoth, and Cirion sent messengers to the Éothéod capital. King Eorl answered the call for help, and rode out with most of the Éothéod to help their allies of old, leaving only a few warriors behind to protect his people. The Riders arrived just in time to help the army of Gondor at the Field of Celebrant, and after defeating the enemy Cirion asked the Éothéod to watch over the depopulated province of Calenardhon.
Three months later Cirion gave Calenardhon as a gift to Eorl and his people, and Eorl swore his Oath of eternal friendship. Messengers were sent north, and the Éothéod completely removed to the plains of Calenardhon.
The Éothéod renamed themselves Eorlingas or 'followers of Eorl', but in Sindarin they became known as the Rohirrim, or Horse-lords, and their country became known as Rohan, the Riddermark.