This page has theories about The Monster form of the Man in Black. For theories about the Man in Black, see this page.

The Smoke Monster is Pure Evil

The monster is not Jacob's brother. When MIB fell into the cave, he was still alive. He removed the cork just like Desmond did in The End. Doing this released the Smoke Monster. The Smoke Monster then scanned MIB, took his memories, and killed him. The Smoke Monster was able to take the form of the dead and chose to take the form of MIB because he knew it would help him. By pretending to be MIB, he could get in close with Jacob. (He later says that he took the form of John Locke because it would help him get in close with Jacob.)

The Smoke Monster is evil incarnate and came from Hell. (When Desmond removed the cork, the red light, heat, and steam seemed similar to common depictions of Hell). Many of the characters said that if the Monster left the Island, it would bring about the end of the world. I believe that the Monster took on some of the traits of the people he scanned. This is why we saw him saying “Don’t tell me what I can’t do” (stolen from Locke) and wanting to get off the Island (stolen from MIB).

If you accept this premise, it seems that the Island was a test set up by God to see if anyone could defeat the Smoke Monster. Those who could defeat him would reach Heaven/Nirvana/Paradise. The Smoke Monster can also be seen as a metaphor for one's own personal demons. When one conquers his/her personal demons, he /she will reach Heaven/Nirvana/Paradise.

History

An older Smoke Monster predated the Man in Black

The Temple hieroglyph shows the smoke monster with Egyptians. Though the Egyptian and Roman civilizations overlapped (e.g. Cleopatra and Marco Antonio) the Man in Black was born (and thus transformed) well after the Egyptian structure. The older Monster helped Mother destroy the Roman camp. This is how Mother knows entering the cave is a fate worse than death.

The Egyptians predicted the smoke monster's birth

Their mural, like Locke's monster attack drawing, depicted a scene they'd never seen.

Abilities/behavior

The Monster represents the past

The monster is a metaphor for people's troubled pasts. The more morally questionable actions a person has committed in the past the larger the monster will be for them. When you run from these past events, they will eventually catch up to you. But if you face your problems or the Monster and come to terms with them, they seem less daunting and scary. It is not necessarily the monsters purpose to make you face your past, it simply derives it's power through these terms.

Mr Eko stares the monster down and it slowly shrinks and then runs off. When he is drinking from the stream, the monster sneaks up on him, but he quickly turns and faces it and it slips away. The monster had never been seen so large as when it attacked Keamy and his men, who had committed many immoral acts as mercenaries.

It manifests as whole, dead bodies

The Monster can manifest itself as people whose dead bodies remained whole. That's why the Others burn their dead and cast them out to sea. It successfully took the form of Alex, even though Richard buried her. It usually takes the form of those who died on the Island, which explains why Emily Linus's appearance surprised Richard, who's earlier only seen it appear as his own wife.

It is electromagnetism

The Monster is the manifestation of the high electromagnetic fields the island produces. High electromagnetic fields are known to cause hallucinations and "fear cages" that can enclose on peoples psyches. DHARMA has been studying electromagnetism and possibly made the connection that the Monster is made up high electromagnetic fields, so they built the fence around the barracks to protect themselves.

The spectral analysis of the sound made by the sonic barrier, one of the frequency bands corresponds to 972 Hz, which is the average resonant frequency of the human skull. The barrier could be a means of not allowing the Monster to "get into the mind" or "skull" of its target.

The leader summons the monster

Only the Island's leader can summon the Monster.

It drags people to EM pockets

The Smoke Monster dragged people into Cerberus Vents (which all lead to a pocket of electromagnetic energy, like all the wells on the Island do) because it was trying to use them to "unplug" the energy pockets there. Similar to what he wanted to do at the Source, which was the strongest of all the EM pockets. Since Smokey had no way to access the Source back then it had to try to make do with the "weaker" pockets to accomplish its goal of leaving the Island. Didn't work, though.

The reference "a mouse does not rely on just one hole" that is on the blast door map (assuming that 'Cerberus' is the smoke monster) is referring to the Cerberus Vents, or CV I-CV IV. These include the hole that the monster almost dragged Locke into.

There Are Two Monsters/It appears differently to different people

As referenced when Locke and Eko first encounter the monster, the monster seems to appear as light to those two. They describe it as "beautiful" and "a great light" yet obviously it appears as dark smoke to everyone else. So perhaps this is Jacob as an unMonster scouting candidates on 815 or Locke and Eko are somehow inherently different. Q: If it is the latter case, what is it about Locke and Eko that makes the monster appear differently to them?

COMMENTARY S01 DISC1 E04: Terry O'Q. (when talking about the first smoke monster encounter) "-I was told to imagine it was the most beautiful thing I had ever seen."

A: Locke and Eko both came to the Island without fear, either losing it when they arrived (Locke) or conquering it before they arrived through horrific life experiences (Eko). This lack of instinctual fear was potentially problematic for the plans of the smoke monster. Smoke Monster knew that both men saw the Island as a place of renewal, beauty, and miracles, so it appears as something wonderful once to each man to gain confidence and assurances, to show the Island is a place of magic. The Smoke Monster was running the con from the start, it knows how to manifest itself to others so-as to get what it wants.