Now, this theory is open to interpretation. In The Cost of Living, Locke describes to Eko, that his encounter with the Monster was seeing a 'bright light'. This bright light is where your interpretation starts: if you believe that Locke saw the normal, black smoke monster we've all seen before (and he just saw it differently to everyone else) then this theory isn't for you. But if you believe he saw something different, read with an open mind.
First of all, I will put to you: If this bright light was just the normal black smoke monster, then why wasn't it shown on the episode? Why would the writers leave out the smoke monster here?
My theory is that Jacob is the bright (white?) smoke monster, and his Nemesis is the black smoke monster. Obviously, we do not know why they are smoke smoke monsters (nor if this theory is incorrect do we even know what the smoke monster itself is), but the smoke form is their original form.
The two men seen portraying Jacob and his Nemesis (in The Incident) are just two forms that they have taken from people who have died on the island. They can change form whenever they like, but the black smoke monster is out and about more because he is not in hiding (unlike Jacob/white monster).
Questions though such as why the white smoke monster went to see Locke and why the black smoke monster was going to drag Locke into the tree hole, however, are unknown, like everything else on the show!
P.S. To those who doubted my Locke Is Dead theory (http://lostpedia.wikia.com/wiki/User_blog:Daneveritt/Locke_Is_Dead), I was RIGHT! ;)
Dan