In the old north of Middle-earth lived a proud race of wild Horses, long-lived, wise and fleet of foot. They were called by men (particularly the Rohirrim) the Mearas. The legends of men said that their ancestors had been brought from the West by Béma, their name for the Vala Oromë.
One of these was captured as a foal by Léod of the Éothéod. It grew into a strong white horse, but when Léod tried to mount it, it threw him and killed him. Léod's son Eorl took the horse himself, naming him Felaróf. Felaróf was one of the greatest horses to have ever lived, and was said to understand the speech of Men. He carried Eorl when the Éothéod rode south to Rohan, and there sired a race of horses nearly as great as himself.
These were the Mearas, noble horses that lived as long as a Man, and had extraordinary strength and intelligence. Throughout their history, they would only allow themselves to be ridden by the Lord of the Mark or his sons. This long tradition was broken by Gandalf, who managed to train the greatest of the Mearas of his time, Shadowfax, and rode that mighty horse throughout the War of the Ring.
The word Mearas comes from an Anglo-Saxon word meaning simply 'horses'. Its singular form would be mearh, but no character in The Lord of the Rings ever uses that word.