[CSteefel]

What I find confusing is that Gandalf says that the old man was not him, but the appearance of the old man is also (apparently) linked with the horses running off. As pointed out, however, the horses were in fact not scared away at all, but were overjoyed to encounter Shadowfax. These two events seem to be linked, but one points to Gandalf (or at least to his horse), the other to somebody other than Gandalf.

This whole issue is made a bit more ambiguous by he fact that there is some blurring of Gandalf's and Saruman's roles as the White Wizard when Aragorn and company encounter him later in Fangorn...

Still, one has to accept Gandalf's emphatic statement that what they saw was certainly not him. In addition, there is no reason why Gandalf would not have revealed himself to the company at this stage if it was him. So the phantom of Saruman seems most reasonable...