Who was the old man?

[The Might]

Well it said Saruman was asking himself whether the Ring had been used in the battle or if Theoden had it and had learned of its powers and so returned to Isengard in order to double and treble his efforts against Rohan.

However, it still makes no sense. Of course it is plausible that Saruman being in the area came along said "Boo!" to the horses and left, but I just can't believe it.

Maybe it was a phantom after all:

Say, are you not a wizard, some spy from Saruman, or phantoms of his craft? Speak now and be swift!'
'We are no phantoms,' said Aragorn, 'nor do your eyes cheat you.

'I wish to see him and learn if he really looks like you.'
'And how will you learn that, Master Dwarf?' said Gandalf. 'Saruman could look like me in your eyes, if it suited his purpose with you. And are you yet wise enough to detect all his counterfeits?

So it seems that actually there were phantoms of Saruman's craft and as Gandalf says Saruman could disguise himself.

Such a difficult topic...