Who was the old man?

[Kitanna]

I always assumed it was Saruman or at least his phantom. If Gandalf had sent a phantom of himself he would have told the three hunters.

I always forget about this part until I read it, then for a few chapters I'm perplexed until I forget about it and the process starts anew when I reread.

afterwards caused the horses to flee hoping to weaken the three Hunters that way.

I seem to recall the horses may have fled not out of fear, but rather they met Shadowfax.

Whether they fled at first in fear, or not, our horses met Shadowfax, their chieftain, and greeted him with joy.

If Saruman was a phantom he had made this phantom with such perfection it was taken from flesh and blood which would mean he'd have to make a lot of noise or some great display to scare war horses off. I think rather Saruman (or phantom) was just out wandering about like he was prone to do.

Remember the words of Eomer: he walks about like an old man hooded and cloaked.