The Smoke Monster took the form of it in order to manipulate Locke. edit: he says he was rammed of the road and hauled into an ambulance. The ambulance was likely the Others who drugged him and brought him there. The smoke monster taking the form of him (which never happened because he wasn't dead yet?) would answer a transportation question.
The truth is we don't know yet. The only explanation is by Ben saying that the island is a 'Magic Box'.
HE most likely flew on geese strapped together. much like johnny depp. Cept' with seaturtles, mate.