In the 14th-century novel Romance of the Three Kingdoms the Warlord Gongsun Zan is described as having some of his riders mounted on White Horses because nomadic tribes (?) he fought revered them as holy and thus would flee rather then kill the horses. Now this is so well known it seems that he is always statet to be the "White Horse General". I also always thought that this is a historical fact but today I read his translated Sanguozhi biography (which are the historical record of the Three Kingdoms era) online and it never even mentioned any White Horses at all. My question would be, is there any historical record, or any other volume of the Sanguozhi that mentiones the White Horses in association with Gongsun Zan or is this actually just another invention for the Romanticised Novel?
Both the Hou Hanshu and Wang Can's Records of Heroes mention that Gongsun Zan had a personal companion force of mounted archers riding white horses. When Gongsun Zan was fighting against the Wuhuan and/or Xianbi in Liaodong in ~186 his enemies could thus recognize him and his forces by those white horses, possibly even referring to him as the "White Horse General" or other epithets. de Crespigny's Biographical Dictionary of the Later Han also mentions them, presumably drawing from the same two sources.
Now, there are historical references to white horses being at least a little bit special. E.g. from de Crespigny's To Establish Peace / Zizhi Tongjian:
Jiao Chu proclaimed himself Inspector of You province, and persuaded the civil officials in all the commanderies to abandon the Yuan and transfer their allegiance to Cao Cao. He drew up his troops, several ten thousand in number, and sacrificed a white horse to confirm the covenant, giving orders that, "Anyone who turns away will be killed." None dared look up, and each in turn smeared the blood on their mouths [to accept the oath]
However, as far as I know there is no specific historical reference about the white horses being especially sacred or revered by the Wuhuan or any of the northern peoples refusing to fight against opposing forces because they had white horses. The Hou Hanshu suggests the Wuhuan were wary of fighting "the general with white horses" but not because the horses themselves were special, just because Gongsun Zan had been successful against them in previous battles and the white horses made it easy to tell his forces apart from other Han forces.