Thralls horde is a small part of the former horde, mainly from those of Doomhammer and the more honorable tribes. With the less honorable ones dying with Gul’Dan. The other parts is darkspear trolls, which were outcasts from troll society and bloodtotem tauren which were nearly hunted to extinction.
The old horde did not have tauren. And their troll allies were amani, not darkspear.