The Rohirrim, why horses?

[Inziladun]

Originally Posted by Goldberry123

I have many unanswered questions questions, for instance... Why does Shadowfax allow Gandalf, rather than anyone else, ride him?

The answer to that must lie in Gandalf's nature as a Maia, sent by the Valar. The Vala Oromë could have been at work there, providing the leader of the fight against Sauron with the one horse that could bear him swifter than any other, and stand up to the terror of the Nazgûl.