Rohirrim !! (The Lord of the Rings)

The Rohirrim are descended from the Éothéod, a race of Men who lived in the vales of the Great River Anduin, but that removed to Calenardhon, which was granted them in perpetuity by the Ruling Steward of Gondor, Cirion in reward for the assistance that they offered Gondor at a time of great need. As a result, they were lifelong allies of Gondor bound by the Oath of Eorl. At that time Calenardhon was renamed Rohan (Horse-land) after their many horses. By the Rohirrim themselves Rohan is usually called The Mark.
The terms Riders of Rohan and Riders of the Mark are commonly used and refer specifically to their mounted soldiers. The former is a chapter title in The Two Towers. The King's Riders were specifically the Riders who formed the bodyguard Of the high king

Rohirrim recruit
Rohirrim recruit
The Dúnedain of Gondor believe that the Rohirrim were distantly related to them (having descended from the Atanatári of the First Age) and describe them as Middle Men, that being inferior to the Númenóreans in both culture and descent, but superior to the Men of Darkness who had worshiped and served Sauron.
The Rohirrim had had contacts with Elves in their ancient history, and knew of Eru, but like the Dúnedain they did not worship him in any temples. They seem to have highly valued the Vala Oromë the Hunter, whom they called Béma.

They are ruled by a line of kings descended from Eorl the Young, who had first brought them to Rohan, and in time of war every able man rode to meet the Muster of Rohan.