J.R.R. Tolkien used Old English as the basis for the fictional language Rohirric.
In The Lord of the Rings, the language of Rohan is represented through Old English words, names, and forms. This reflects the relationship Tolkien wanted readers to perceive between the Rohirrim and the people of Gondor: Rohirric functions as a distinct language in the story, while the text presents it through a historical language familiar from English tradition.
Names such as Éomer, Éowyn, Théoden, and Meduseld draw on Old English inspiration. Tolkien was a philologist and a specialist in historical languages, so his linguistic choices were part of the world-building rather than decorative inventions. Rohirric should not be confused with Sindarin or Quenya, which are Elvish languages constructed from different linguistic ideas and histories.