The smoke monster in Lost was originally intended as a mechanical contraption built by the Dharma Initiative. Lost's island offered an abundance of mysteries from the outset of the groundbreaking ABC series, but one of the earliest, and most paranormal, was the presence of a monster that could knock down foliage and leave Oceanic 815's pilot mangled and bloodied halfway up a tree. Lost season 1 eventually reveals the form of this threat to be a cloud of black smoke and, as the series progresses, viewers see the smoke monster attack humans throughout the island's history. Seemingly invincible, the smoke monster possesses incredible strength but can be repelled through the use of sonic fences or ash circles.
Eventually, the smoke monster is confirmed to be the Man in Black - an ancient inhabitant of the island that was cast into the Heart of the Island and lost his physical form. As the smoke monster, the Man in Black is able to take on the form of dead islanders and has been waging a centuries-old war with his brother, Jacob, in an attempt to leave the island. However, J. J. Abrams and Damon Lindelof's original vision for the smoke monster was vastly different.
According to the Lost series bible that was compiled after the filming of the show's pilot as a sales pitch to the network, the smoke monster was suggested to have "man-made origins" that linked back to the early idea that led to Dharma Initiative, either as a result of the group's cutting-edge scientific experiments or as a security system designed to protect the island's various Dharma stations. The document also describes the smoke monster as "more machine than animal."
Obviously, these plans were changed somewhere along the way and, reading into the show's development of the smoke monster mystery, it may not have been that early on. Early appearances of the smoke monster came with mechanical sound effects that imply technology and machinery are behind its appearance and the monster is described as "Cerberus" on the ultra-violet blast door map of the island. Cerberus guarded the gates of hell in Greek mythology, suggesting that the original plan to have the smoke monster as a Dharma guardian system was still in place during season 2.
In Lost's fourth season, Ben is seen summoning the smoke monster from a special chamber using some kind of ancient mechanism that some suggested are responsible for the machine-like noises. Once again, this propagates the theory that the smoke monster was still being written as a security system as late as season 4, even if the mechanical origins had been explained away by this point. With the mechanical sound effects, links to Cerberus and the summoning chamber, it seems that Lost's original plan to reveal the smoke monster as a guardian remained in place long after the pilot, even as more mystical aspects had been introduced. Indeed, it does appear that only in the final two seasons was the decision made to turn smokey into both Lost's final villain and the monstrous form of a human character.
In hindsight, this switch of emphasis had mixed results. On one hand, the final smoke monster reveal allowed several mysteries to be cleared up all at once, such as the recurring appearances of dead people on the island. However, it does mean that some mental stretching is required to marry up every detail about Lost's smoke monster into one cohesive story. For instance, Ben Linus' ability to summon the monster had to be retconned, with Ben later claiming the monster only "allowed" itself to be called into action.
Lost is often accused of not having a long-term plan and making up answers to the show's mysteries as it went along. While the change in direction for the smoke monster does somewhat suggest a lack of vision, it could be argued that this particular mystery was built-up to such an extent that a grander resolution was needed than a mere security system, otherwise viewers would've felt disappointed.