Back in Walkabout when Locke was hunting for boar, Michael and Kate hear the "Monster" (which is now known that he is the Man in Black) going through the jungle towards Locke. Locke then "sees" something. What that is, we are not sure of yet. It could have been The Monster. Or perhaps it is someone else that Locke saw, as he has claimed to have seen a "white light" rather than a black smoke.
In the opening scene of The Incident we see Jacob wearing white, and his nemesis, The Man In Black wearing black, which obviously led some people to beleive Jacob is the opposite of MiB(so, a white version of the Monster).
Perhaps The Man in Black wanted to use Locke's body as part of his "loophole" to get Jacob killed as early as season 1 and the reason he didnt get to Locke is that Jacob could have gotten to Locke first to protect him from the Monster, and therefore the white light that Locke saw was not The Monster(MiB), but Jacob, appearing as his white form.
Evidence of this theory is that in the season 1 finale we see the Monster trying to pull Locke into a hole in the ground. The reason for this could have been MiB trying to get Locke for his body. He just didn't succeed until season 5.