PREMISE: Two smoke monsters....One black and the monster of the dead, only able to assume the form of those dead and unburied. One white smoke monster that can assume the form of the living.
Locke: "Two sides - One dark and one light"
Episode 101 - Pilot - First night on the island....Smoke monster approaches through the trees and EVERYONE is looking to the left at the noise. BAM....Second noise and EVERYONE turns 120 degrees to the right to look in the same spot, though the camera angle never shows the treesa that direction. BAM....Noise to the left again and EVERYONE turns back to the left and we see the monster in the trees retreating. In other words, in the first 30 minutes of the entire show, we are shown (though had no clue in season one to consider this fact) that there is a black smoke monster that comes for the unburied so he can assume their form (e.g., Christian, Yemi, Alex, to name a few). The second smoke monster intercepts in a way to scare off the black monster.
Episode 101 - Pilot - Fuselage Scene - On the beach, a monster is heard by those on the beach, and the trees shake. Three minutes later the scene cuts over to the fuselage where the pilot is ripped from the cockpit. We are also told before Jack, Kate and Charlie depart for the fuselage that it is roughly 3 miles away. So 3 miles and 3 minutes means that the smoke monster on the beach would have to move 60 MPH to travel three miles in three minutes. So far, we have not seen the monster move remotely near that speed. This suggests there was one smoke monster on the beach, and one at that the fuselage.
Episode 104 - Ex Deus Machina - Locke is in the jungle alone....The trees rustle and a monter approaches...It rises up and looks down, but we never see the monster and it never hurts Locke. However, Locke later declares he has looked into the eye of the island and it was beautiful. The look Locke had up at the monster was the same look as that of one who was being scanned by a monster.
Season One (generally) - After Episode 104, Locke appears out of no where at least a half dozen times, surprising those he approaches because he was so quiet. For example, in one scene, Sayid is studying a map when suddenly Locke appears without making a single approaching sound of walking or running. However, Locke is slightly out of breath, holding a knife, and sweating. Locke then offers words of wisdom to Sayid, and gives him a compass, stating, "I don't need it anymore," smirks and walks off. The point is, white smoke monster Locke (having scanned the form of the living) is assuming the form of Locke. More evidence by better example....
Episode 106 - WHITE Rabbit - Jack goes chasing after Christian (the black manifestation of Christian since Christian was never buried and the black smoke monster can assume the form of that body now). As Jack chases, he enters a field in the afternoon and there is white fog or haze moving over that field. Being new to the viewing of an island, the watcher never considers that fog should not be there in mid-afternoon - Morning maybe. Jack proceeds to one final chase of Christian (black smokie leading Jack to possible death) when Jack stumbles over a cliff. Who should appear from no where (or at least from the white haze / fog) but white smoke monster Locke. Notice how Jack and Locke fall to the ground when Locke saves him and laughs. Very bizarre directing to catch our eye and indicate something odd.
Later in season 5 there is an emphasis after Sawter kills two Others in 1974 that they MUST bury the dead. It is because the black smoke monster will assume their form if left unburied, and the lifing will be confused by the manipulations of the fake people.
Lastly, all through season one and 2 in the flashbacks, there are many instances of white smoke appearing at times that seemed out of place, but that the directors emphasized. Like in the flashback in the field at night as Kate and her childhood friend are burying the lunch box time capsule under the tree at night.....There is a focus on lighting smoke / haze over the tree in the night time...That should not have been there, but the directors highlighted it with the lighting.
There are a dozen more instances to high light this point: Two sides: One light (white smokie), and one dark (black smokie), and one original Locke. Thus, three Lockes across the show.