The monster is actually much more versatile than most people recognize it to be. The monster comes in many, many different forms and its purpose is simply to serve as an obstacle for the LOSTies to overcome in order to achieve their objectives.

The 'game players' can choose which version of the monster will appear. This may be done through a series of prompts by the video game to the 'game players'. The monster then appears as a polar bear, shark, boar, whispers, hallucinations, sea urchins, rain storms, the black smoke monster, etc. I have included the heroin as Charlie's personal version of the monster. It is possible that the game itself may insert a random version (or even more than one version) of the monster into some episodes. For instance the Others and the BOATies may be introduced by the game itself, rather than by the 'game players'.

The Others are just one of the many versions of the monster. However, once they are introduced, it is possible that they may actually remain in the game until that particular level, if not the entire game, is completed.

Watch this Season 4 promo where the statement is made that the monster can take human forms.

An 'innteresting' thing can be found on the LOST Season 1 Soundtrack. Take a good look at the index of the soundtrack and the title of track 25. It is "Monsters are Such Innteresting People".   Click the picture to see a larger version

There is also a curious exchange between Jack and Tom/Mr. Friendly in S3E9. Tom/Mr. Friendly asks Jack "What kind of people do you think we are?" and Jack replies "Oh, I don't know Tom. The kind of people that would take a pregnant woman, that would hang Charlie from a tree, would drag our people out of the jungle, would kidnap children. That's what kind of people I think you are."

And there are times when The Others have been referred to as animals:
In S1E15 Charlie says about killing Ethan "I wasn't going to let that animal anywhere near her again"
In S2E6, Ana-Lucia says "They came the first night that we got here. They took 3 of us. Nothing happened for 2 weeks, then they came back. They took 9 more. They're smart, and they're animals, and they could be anywhere at any time.
In S2E20, Michael says "Then let me do it. They're animals. I've seen these people and they are... animals"

In the January 17, 2006 podcast, when discussing the monster, Lindelof says "...this week we saw the latest incarnation of the monster". He was referring to the black smoke version of the monster that Eko encounters in S2E10.

In the May 26, 2006 podcast, Lindelof and Cuse say that the audience should not assume that the writers abandoned the monster in Season 2 . That "there is a good chance that the audience saw the monster more than they think. But that the audience just didn’t recognize that it was the monster."

S4E10 also gave us the fact that even though the smoke monster attacked Keamy's men, they all seem to have survived. When asked about this in an interview on Jimmy Kimmel after the show aired, Carlton Cuse stated that "The monster has different rules for different people". Cuse also answers Rose's question as to why Jack got sick on an island that actually cures people, when Cuse answered he said that "Jack gets sick because he is not in communion with the island". This is the first indication we have a possible answer for why Ben got his cancerous tumor and ties in with Richard Alpert's statement to Locke in S3E19 that the Others are concerned that Ben is too obsessed with the pregnancy issues.

Below is my listing of the many versions of the monster by episode #'s.

Season 1

Jack hallucinates his father
The first version of the monster that we see is the one that that knocks down the trees near the beach, then it causes the rain storm, passes over the window as smoke or a shadow and pulls the pilot up through the windshield, kills the pilot and chases Kate into the banyan trees. Listen to the monster
One of the LOSTies says "It didn't sound like an animal, not exactly."

Polar Bear
During the crash scene, as Kate is trying to get the handcuffs off her wrists, you can hear the distinct noises that the monster near the beach makes
Heroin

Joanna drowns in a riptide
Jack hallucinates about his father

Danielle tells Sayid she hears whispers
Sayid hears whispers
Others- Danielle tells Sayid about the Others
Ethan Rom/Other
Booby Trap- Sayid hits a trip wire for a snare trap

Rain Storm
Others/Ethan attacks Claire in her sleep with a syringe
Others/Ethan kidnaps Claire and Charlie
Dreams/Hallucinations- Claire has nightmare with Locke in it
Heroin

Whispers- Sayid tells Sawyer about the whispers he heard
Other/Ethan- hangs Charlie
Other/Ethan- meets Jack
Bird screeching sound that turns into what might be whispers just before Jack falls down the hill
Rain Storm

Season 2

Shark attacks the raft

Ben/Fake Henry Gale
Rain Storm

S2E23 & S2E24

Season 3

Others
Whale calls can be heard as Jack asks Juliet if he is in an aquarium
Jack hallucinates his father saying "Let it go" over the intercom

Others
Nikki asks what happened to Eko and Locke replies "Must have been an animal, maybe one of the bears"
Whale Noises when Kate and Sawyer are cuddling
Rain Storm
Sayid asks Locke what killed Eko and Locke says "Folks back at the beach call it the monster"
Heroin

Others
Rain Storm

Others
Animals- cows, horse, chicken and Mikhail Bakunin/Patchy's cat Nadia. Listen to the March 26, 2007 podcast and the discussion about the relationship between Kate's horse, Sayid's cat and Christian Shepard. Also see Instructions/Rules for more about Nadia.

Rain Storm
Monster that knocks down trees accompanied by the noise that it makes
Black Smoke Monster
Boar
Others- Sayid remarks "It's like 50 people disappeared into thin air"
Mud Pit- Kate and Juliet fall into a mud pit
Perimeter fence around Otherville
The ticking sound of the monster makes as it retreats is heard just prior to the flashes being seen. Listen to it.

Others
Anthony Cooper calls Ben/Fake Henry "Bug eye"
Naomi and the BOATies

Others/Hostiles
Jacob is seen briefly and he says "Help Me" to Locke when Locke and Ben/Fake Henry are in Jacob's house. jacob seems to look quite a bit like Christian Shepard
Whispers- Ben hears whispers twice, just before he sees his mother and then just before Richard Alpert appears
Hallucinations - Ben sees his dead mother Emily outside his window and near the sonic fence

Others
Rain Storm in Charlie's Flashback
Naomi and the BOATies

Naomi, George Minkowski, Daniel Faraday and the rest of the BOATies
Others
Rain Storm
Whispers - Hurley hears whispers when he finds Jacob's cabin
Jacob/Christian Shepard - When Hurley finds Jacob's cabin he looks inside and sees a figure rocking in the chair, an enhanced view of the photo is shown above. A larger view of it is available here. The monster has taken on the form of Christian Shepard. An eye appears suddenly from the inside of the cabin that looks just like the eye seen in S3E20. As Hurley tries to run away from the cabin, it suddenly reappears in front of him and then it disappears when he insists that it is not there.
Hallucinations- Hurley hallucinates Charlie while he is in the convenience store, in the police station and at Santa Rosa Mental Hospital

Smoke Monster- Ben enters the secret room off his secret closet and returns covered in soot, moments later, the smoke monster appears and attacks Keamy's men and grabs one man. The smoke monster is accompanied by its usual sounds, but also appears to be able to be able to shake the house, race like a train and also swirl like a tornado. There are also flashes of light as the smoke monster roars in like a train and there are flashes of light when the smoke monster is attacking Keamy's men in the jungle.
BOATies
Others- Charles Widmore says to Ben "I know who you are boy, what you are"
Nightmares- Charles Widmore gets nightmares

Hallucinations -Jack sees his father in the hospital lobby when talking to Ms. Berenberg and then later when taking the battery out of the smoke detector. Hurley tells Jack about seeing Charlie, that Charlie told Hurley that "You're not supposed to raise him Jack" and that someone will be coming to see Jack too, soon. Later Jack sits outside and stares at the bench where Hurley says Charlie visits him
Hallucinations- Claire sees Christian Shepard holding Aaron
BOATies
Others
Birds Chirping- There are birds chirping that serve to cover over the sounds that Aaron makes to help hide him from keamy and his men. This could indicate that the monster is working to protect Aaron