The fictional language and naming style of the Rohirrim are based on Old English in Tolkien’s writings. Tolkien portrayed the people of Rohan as horse-lords whose culture and vocabulary echo Anglo-Saxon England. Their names, titles and place-names therefore resemble forms from Old English.
For example, Éomer, Éowyn and Théoden use forms inspired by Old English, while “Mark” reflects the Old English word mearc for a borderland. Tolkien’s choice creates a linguistic relationship between the Rohirrim and the Anglo-Saxon tradition that he studied professionally.
The Rohirrim do not literally speak modern English within the story. Tolkien presents their speech as a translation convention: readers see Old English-derived words representing another language in Middle-earth. This differs from the Elvish languages Quenya and Sindarin, which Tolkien constructed with different linguistic inspirations, including Finnish and Welsh.