From The Visitors’ Guide To Wiltshire, circa 1973:
Above Westbury stands the oldest white horse [in Wiltshire].
It is said to have originally commemorated King Alfred’s victory over the Danes in the ninth century.
It was a carthorse until, in 1778, “a wretch by the name of Gee”, steward to Lord Abingdon, remodelled it – more elegantly he thought.
It is now 180 feet long and the height to the shoulder is 107 feet.
Originally, in each of the horses [in Wiltshire] there appear the sign of the Crescent Moon, which has no equine significance, so they may have all been modelled on a more primitive emblem.