The language J.R.R. Tolkien created for the Riders of Rohan in The Lord of the Rings is Rohirric.
Rohirric is the language associated with the Rohirrim, the horse-lords of Rohan. Tolkien generally represents it in the narrative through Old English forms rather than presenting a complete, separately documented constructed language. This choice reflects the fictional relationship between the languages of Rohan and the Common Speech.
In Tolkien’s internal history, the Rohirrim’s language is related to the language spoken by the ancestors of the Men of Dale and by the Northmen. The use of Old English gives Rohan a distinctive cultural and linguistic identity while signaling an older relationship with the people of Gondor.
Rohirric is therefore different from Quenya and Sindarin, which are Elvish languages, and from Khuzdul, the secret language of the Dwarves. Terms such as Éored and Éomer are connected with Rohan’s linguistic presentation.