1 - The Stonemaul Clan of Ogres
The Stonemaul were conquered by Rexxar in WC3, during the Founding of Durotar. They’ve been part of the Horde ever since.
2 - The Revantusk Tribe of Forest Trolls
The Revantusk Tribe was one of the smallest tribes of the Amani empire, and when Thrall established the New Horde, they were interested. However, they still remembered the old Horde and that made them too wary to join the Horde outright.
But during the Cataclysm, that stance changed, and they joined the Horde.
“We are proud to call ourselves the last tribe of Forest Trolls to serve the Horde.”
-Elder Torntusk
3 - The Taunka of Northrend
The Taunka, like everybody else in Northrend, had a bit of a Scourge problem back in Wrath of the Lich King. During the quest “Blood Oath of the Horde”, a group of Taunka join the Horde to fight back against the Scourge.
4 - The Grookin Tribe of Hozen
The Hozen of the Jade Forest joined the Horde after a Horde adventurer beat them up, then saved them from some crocs, then gave them some fish, then helped them fight their enemies, the Jinyu. Yeah, it was a weird adventure lol
5 - The Broken Tribes
The Broken are part of the Draenei people. The ugliest part. But they are there, always supporting their prettier brothers and sisters in the Alliance.
6 - The High Elves of the Silver Covenant
I know… Let’s move on…
7 - The Frostborn Dwarves of the Storm Peaks
During the campaign against the Scourge, the Explorer’s League found and befriended some Frost Dwarves in Storm Peaks, who were led by a regular Dwarf. Turns out that regular Dwarf was Muradin Bronzebeard, who lost his memories after Arthas exploded a block of ice near him to grab a fancy sword. Muradin has returned to his original people, but the Frostborn still love and respect him.
8 - The Pearlfin Tribe
The Pearfin Tribe of Jinyu were wary of strangers for thousands of years after a betrayal from the Hozen caused them to lose the Throne of Pandaria. Then they joined the Alliance because the spirits said so and also the Hozen had joined the Horde.