The language spoken by the Rohirrim in J.R.R. Tolkien’s legendarium is Rohirric.
Rohirric is the language associated with the people of Rohan, the horse-lords of Middle-earth. Tolkien usually represents it through Old English forms in The Lord of the Rings. This literary choice makes the Rohirrim’s speech feel related to, but distinct from, the Common Speech used by many other characters.
Rohirric is not simply identical to Old English within the fiction. Old English is Tolkien’s translation convention for the language of Rohan, allowing modern readers to recognize a historical linguistic relationship while preserving the imagined setting.
The language should also be distinguished from Westron, the Common Speech, and from Sindarin, an Elvish language. Tolkien’s use of real historical languages as representational equivalents is one reason the linguistic texture of Rohan differs so noticeably from that of Gondor and the Elves.