We have no definite references to the White Horse upon Green preceding the War of the Ring, with the oldest potential exception being references to shields that carried the device (and one of these, used by Gimli, was said to date back to the time of Théoden's father Thengel). Beyond this, all we can say for certain is that it was used as a banner by the Rohirrim at the time of the end of the Third Age, but it is implied that it was rather older than that. The natural assumption would probably be that it dated back to the time of the foundation of Rohan in III 2510, though this is not stated specifically. Indeed, the emblem of a white horse on a green field would have fitted the forebears of the Rohirrim equally well, and it is not impossible that the symbol might have originated among the Men of the Éothéod, or perhaps even the Northmen of Rhovanion long before the foundation of Rohan.

In an abstract sense, the White Horse was a natural symbol for the Rohirrim, for whom horses formed a vital part of their culture. It is not impossible, however, that the White Horse upon Green had a more specific reference. It may be related to the extraordinary horses known as mearas that the Rohirrim bred, or perhaps even a depiction of Felaróf, the white steed of Eorl the founder of Rohan.