Théodred

Théodred was the only son of Théoden and was his heir before his death.

History
Théodred was born to Queen Elfhild and King Théoden. After the death of his aunt Théodwyn and uncle Éomund, his cousins, Éowyn and Éomund, were adopted by Théoden. The cousins were raised together and practically siblings.

He once owned a horse named Brego, who would later be given to Aragorn.

War of the Ring
Théodred was wounded in battle at the Battle of Isen. Éomer was sent to search for Théodred. He found him, but barely alive.

Though Éomer found his cousin alive and took him back to Rohan to be nursed back to health, Théodred died of his wounds, which caused grief to that of Théoden, Éowyn and the people of Rohan.

Théodred was later buried with his other forefathers and Éomer was made Théoden's heir.