Now here's a promo pic from the Lost season 5 finale that shows Jacob sitting in his rocking chair in his cabin. There was no such seen in the episode itself.
And here are a couple of shots from season 4 of "Jacob." Doesn't look like the same person. Why would Locke see an older, bearded Jacob, when the one we see in the finale doesn't seem to be old. Or bearded.
Was Locke seeing an older version of Jacob's as yet unnamed nemesis?
And, of course, we've also seen Christian Shepherd in Jacob's cabin.
My theory is that, for some reason, Jacob's nemesis is body-less. He must appear via other people: Christian, Locke, etc.
I'm also thinking that he may be the Smoke Monster or that he controls the monster.