Calenardhon was the place which became Rohan.
Calenardhon was a wide green landscape in the north of Gondor, but it became depopulated during the Third Age. During the rule of the Steward Cirion Gondor faced a series of attacks by the Balchoth, and Calenardhon was overrun by these evil people. Cirion sent messages for help to Gondor's old allies, including the Éothéod, to help defend Gondor.
After the Éothéod settled in Calenardhon, the land became known as Rohan or the Riddermark.