Brotherhood of the Horse

The Brotherhood of the Horse (aka Knights of Azeroth or Knights of Stormwind ) was the official name of the kingdom of Stormwind's knights before the First War.

First War
The most notable, and only known, member of the Brotherhood was Anduin Lothar, who became a knight and rose to the position of Armsman. They were well respected throughout the realm, and the tales of their fighting prowess were known far and wide.

Prior to the beginning of the First War, the orcish forces who came through the Dark Portal easily slaughtered the human peasants they encountered. They met their first real opposition in the knights of Stormwind, who defeated the unprepared troops of the Twilight's Hammer clan and the Bleeding Hollow clan during the first siege of Stormwind Keep. The knights then rallied the remaining soldiers to temporarily push the orcs back to the Black Morass, where the orcs escaped by hiding in the Swamp of Sorrows.

Toward the end of the conflict, Orgrim Doomhammer specifically targeted an encampment near the center of the human lands where knights and other soldiers were sent to train. Most of the knights were killed fighting in the last battles of the war but took many orcs with them.

Second War
The dark fate of the Brotherhood was a grim news to the knights of Lordaeron, who during the Second War sworn to both avenge their fallen brethren and free their homelands from the grip of the Orcish Hordes.

Many of the knights of Stormwind who fell to the orcs during the First War were transformed into death knights by Gul'dan.

In the RPG
Stormwind proved an overwhelming shock to the Horde. The footmen guarding the city's entrance put up the first fight the orcs had seen, but they still managed to push their way through the gates. By the time they realized that this minimal resistance was a trap, it was far too late. Armored warriors riding beasts of pure muscle and sinew — creatures we know as horses — flanked the Horde's disorganized group, crushing warriors under the hooves of their mounts and striking down even the mightiest of orcs with their lances and blades. The orcs learned to call these mounted warriors knights and cursed the world that had cost them victory for the first time. Shamed, the orcs retreated, hounded by the leaders of the Brotherhood of the Horse every step of the way. Gul'dan concealed the final steps of the orc retreat with a wall of impenetrable fog; this simple spell may have saved the Horde from complete destruction.

Trivia

 * The tower of Karazhan, home of the sorcerer Medivh, contains horse head animal motifs in various ornamentation as a reference to Lothar and the Brotherhood of the Horse.
 * The sword Ashkandi, Greatsword of the Brotherhood is rumored to have belonged to Anduin Lothar. If that was the case, its name would as such be a reference to the Brotherhood of the Horse.

Speculation
Gavinrad was a Stormwindian knight before becoming a paladin and as such he was also probably a member of the Brotherhood.