Rosemary Sutcliff imagined the story behind the magical White Horse of Uffington for her 1977 children’s book (historical fiction) Sun Horse, Moon Horse. It involved the Epidi (meaning “Horse People”), a tribe that had also appeared in her earlier 1954 novel The Eagle of the Ninth. In the author’s note to the original publication she wrote:
If any of you who have read it have also followed the adventures of Marcus and Esca in The Eagle of the Ninth, and think that Lubrin’s people are not very like the Epidi who they found when they went north to rescue the eagle of the lost legion, I can only say that when I wrote that story, I had not read (T. C. Letherbridge’s bokk) Witches. And if I had, I would have made them a slightly different people. Though, of course, they might have changed quite a lot in more than two hundred years.
Earlier she explained the origins of her story, in a book by T. C. Letherbridge and an unusually old White Horse:
Most of the White Horses still to be seen cut on English hillsides have been there since the eighteenth or even nineteenth centuries; but the White Horse of Uffington, high on the Berkshire Downs, belongs to a much older world; nobody knows for sure how long ago it was made, but probably about a hundred years before the birth of Christ. And whereas other horses stand stiff and still on their hillsides, elegant, sometimes, but without any spark of life, the Uffington White Horse is magical; full of movement and power and beauty.
I have always felt that anything so magical must have a story behind it. A long-forgotten story, which I should love to tell. And then one day, reading T C Letherbridge’s book Witches, I cam upon his theory that the Iceni, the great Early Iron Age Tribe who inhabited East Anglia, were also in the Chilterns and the Down Country north of the Upper Thames Valley, until they were forced out by invaders from the south. And I began to get an idea of what the story might be.
Sun Horse, Moon Horse is the result.
Mr Letherbridge believes that the Iceni who were forced out became the Epidi of Argyll and Kintyre—Epidi and Iceni both mean “Horse People”—and if that is true, then it must mean that Lubrin’s people got through safely to their new horse-runs in the north, and in a way my story has a happy ending.