Dunlendings were the ferocious, stunted and vicious men that lived in Dunland, close to Rohan. Also called the Wild Men of Dunland, they have long been enemies of the Rohirrim, becuase they are jealous that the rich lands of the old Númenórean province of Calenardhon were granted by the Gondorians to the Rohirrim instead of them.
In ancient times the peoples who were the ancestors of the Dunlendings ranged over much of what later became Eraidor and Gondor, but they were increasingly driven back by the Númenórean. Offshoots of these peoples survived in isolated places like the hilly country of Dunland or in the White Mountains: thus the Dead Men of Dunharrow are akin to the ancient Dunlendings. Some of these people were absorbed into the population of Gondor, and some stayed in the White Mountains. The Dunlendings themselves are a branch of the folk of the White Mountains that moved north to Dunland. Interestingly, the Bree-landers are themselves actually an offshoot of the Dunlendings, who moved even further north until they reached what became the Bree-land, and were absorbed into Arnor. There are few records of the Dunlending language, due to their lack of a written history and poor oral tradition.
The Dunlendings were employed by the wizard Saruman to attack and raid the cities and settlements of Rohan. Since Gríma Wormtongue had corrupted the mind of King Theoden, the Rohirrim were absent for the most part, and the farm boys and old men who picked up rusty swords proved to be little contest to the ferocious Dunlendings.
Wild Men were also present at the Battle of the Hornburg (Battle of Helm's Deep), as well as an odd breed of Half-orcs, derived from Orcs and the Men of Dunland. They fought viciously against their old enemy, but when Gandalf stormed down the hill with a thousand Rohirrim at his back and his staff shining piercing light into the eyes of the Dunlendings, they dropped their weapons and surrendured. This proved to be a wise decision, as the Orcs who fled were killed by a massive forest of Huorns that blocked the entrance to the valley.
After Saruman's downfall, the Dunlendings retreated back into their homeland and did not trouble the people of Rohan. When the wizard Saruman attempted to take over the Shire, there were a number of Dunlendings with him. However, they were slain or driven away by Hobbits, Saruman died at the hands of his own servant, Wormtongue, Sauron, the basis of all evil, had fallen, and King Elessar took the throne of Gondor, the Dunlendings agreed never to trouble the free peoples of Middle-earth again, and their old and evil power was finally broken.