[Gorthaur the Cruel]

It may be that the reason Shadowfax was able to endure the Witch-king was because his rider was none other than the bearer of Narya, the ring of fire that kindled hearts, rather than his own breed. In the case, of Bill the pony, he was also under rejuvenating effects of Vilya, the ring of air, used to preserve and maintain Rivendell. Why else was Bilbo ale to age still so slowly even bereft of the One? It's because he also dwelt under the influence of Vilya in Rivendell, for a time.