We're not told exactly when in their history the Rohirrim adopted the White Horse as their symbol, except that it was in use at the time of the War of the Ring and continued to be used into the Fourth Age. There is one small piece of evidence for its earlier use: when the Muster of Rohan set out for the War, Meriadoc Brandybuck was given a shield emblazoned with the White Horse, which must surely point to its use for at least some years before the end of the Third Age.

In all likelihood, the design was in use far earlier than this, and the natural assumption is that it dated back as far as the foundation of Rohan. The full emblem of a White Horse on a field of green seems a clear representation of the land of the Mark, and we have no records of any earlier banners to contradict this assumption. Indeed, it is even possible that the White Horse had been used by the Men of the Éothéod, the ancestors of the Rohirrim, for whom horses were an equally important part of their culture, and whose name indeed meant 'horse-people'.

The most obvious interpretation of the symbol of the White Horse would probably be that it was simply an abstract expression of the horses so vital to the lives of the people of Rohan. It may, however, have been intended more specifically, perhaps representing the mearas (the famous and extraordinary horses of the kind to which Shadowfax belonged). It may even represent an individual white horse from history. If so, this would presumably have been Felaróf, the great white steed of Eorl the founder and first King of Rohan.