Hatred of the Centaur is a part of tauren mythology. It is recorded in-game on a wall hanging in the Archdruid's tent in Thunder Bluff.
As the mists of dawn faded and the Age of Memory advanced, the demigod, Cenarius, went his own way through the fields of the world. The Shu'halo were sorrowful at his passing and forgot much of the druidism he had taught them. As the generations passed, they forgot how to speak with the trees and the wild things of the land. The dark whispers from the deeps of the world drifted up to their ears once again.
Though the children of the earth closed out the evil whisperings (sic), a terrible curse befell their roaming tribes. Out of the black lands of the west came a horde of murderous creatures - the centaur. Cannibals and ravagers, the centaur fell upon the Shu'halo like a plague. Though the braves and hunters fought with the Earthmother's blessing in their hearts, the centaur could not be defeated.
The Shu'halo were forced to leave their ancestral holdings behind, and roam the endless plains as nomads forever after. It was held that one day hope would return - and the scattered tribes of the Shu'halo would find a new home under the loving arms of the Earthmother.