Eorl the Young
Éothéod (at the sources of the Anduin)
Aldburg in the Folde
Lord of the Éothéod
1st King of Calenardhon
1st King of the Mark
Dates: III 2485-2545
Parents: Léod, Lord of the Éothéod
Children: Brego, 2nd King of the Mark
"He [Eorl the Young] was so named because he succeeded his father in youth  and remained yellow-haired and ruddy to the end of his days. These were shortened by a renewed attack of the Easterlings. Eorl fell in battle in the Wold, and the first mound was raised. Felaróf was laid there also."
'Eorl vowed that he would avenge his father. He hunted long for the horse [his father's killer], and at last he caught sight of him; and his companions expected that he would try to come within bowshot and kill him. But when they drew near, Eorl stood up and called in a loud voice: "Come hither, Mansbane, and get a new name!" To their wonder the horse looked towards Eorl, and came and stood before him, and Eorl said: "Felaróf I named you. You loved your freedom, and I do not blame you for that. But now you owe me a great weregild, and you shall surrender your freedom to me until your life's end."'
"...Cirion, Steward of Gondor, sent north for help; for there had been long friendship between the Men of Anduin's Vale and the people of Gondor. But in the valley of the River men were now few and scattered, and slow to render such aid as they could. At last tidings came to Eorl of the need of Gondor, and late though it seemed, he set out with a great host of riders.
"Thus he came to the battle of the Field of Celebrant...There the
northern army of Gondor was in peril. Defeated in the Wold and cut off from the south, it had been driven across the Limlight, and was then suddenly assailed by the Orc-host that pressed it towards the Anduin. All hope was lost when, unlooked for, the Riders came out of the North and broke upon the rear of the enemy..."
"Cirion, therefore, in reward for his aid, gave Calenardhon between Anduin and Isen to Eorl and his people; and they sent north for their wives and children and their goods and sealed in that land. They named it anew the Mark of the Riders, and they called themselves the Eorlingas; but in Gondor their land was called Rohan, and its people the Rohirrim (that is, the Horse-lords). Thus Eorl became the first King of the Mark, and he chose for his dwelling a green hill before the feet of the White Mountains that we're the south-wall of his land."
Lord of the Rings: Return of the King Appx. A Part II The House of Eorl