In the movie, The Hobbit: The Unexpected Journey, Smaug has 4 legs. If you look closely in this scene where he crawls into the mountain, he has 2 fore legs and 2 back legs.
In the next movie, The Hobbit: The Desolation of Smaug, he has only back legs.
Why were the fore legs removed?
Was it due to an oversight? An error in CGI?Or was it because the book said he had only 2 legs?
Blame Benedict Cumberbatch, from MTV news:
"Originally, the dragon we envisioned was bigger. The idea was to get the fear through his bulk. In fact, if you go back and look at the first film and the scenes that he was in, he was actually a four-legged dragon because we just had him stomping through Erebor in all of those flashback scenes," Letteri said. "But we realized that once you saw him performing — we especially got this from watching Benedict perform. He got down on the grown [sic] and starting slithering around like the way Tolkien described Smaug in the books, which is as a big worm. Once we saw Benedict doing all of that, we realized you can't have him be this four-legged creature with wings on him back, he needs to be two legs and his wings need to be his arms properly, as you would expect a creature to be like a bat or a bird."
This answer starts by clarifying some terms, then goes through some background information and ends up answering the question. The overall answer was improved considering the discussions in the comments.
Dragon is often depicted, in both western and eastern cultures, as a serpent-like legendary creature, winged, four-legged, and capable of breathing fire.
Wyrm is a huge limbless and wingless dragon or dragon-like creature.
Wyvern (Serpe) is a draconian creature possessing wings, only two legs and usually a barbed tail.
Looking at Tolkien's artwork (he was also a great artist btw)... the following Thror's map of the Lonely Mountain was drawn by Tolkien for the first edition of The Hobbit in 1937 and we can see slim dragon with 4 legs
The following is also Tolkien's orignal artwork but was not used in the original impression of The Hobbit, 1937, which included no coloured illustrations (this painting appeared in the second English impression of the same year). Here we see also a dragon (if we look closely we see the left rear leg)
The firm behind the design of the dragon was Weta Workshop (find images about its creation here). From reading their projects I got the impression they weren't the one's behind the first Smaug, but then found the following image where some team members are discussing the initial design (which removed the initial doubts)
So, from the Tolkien's sight, he imagined Smaug as a dragon looking like a serpent in both his drawings.
In the same link the Weta Workshop's image was found, we can read
Most notably, Smaug was apparently a four-legged western dragon in the first film, but to follow the description in the book, referring to him as a "wyrm", he was redesigned to be a more snake or bird-like wyvern-type dragon in the second film. In the Blu-ray extended edition of the first movie, his forelimbs were changed to winged arms. Many other parts of his body have also been altered from what he looked like in the first movie.
Also, from Steve Harrington's quote
«Tolkien described Smaug in the books, which is as a big worm»
So, indeed, Smaug was a dragon in the first film, which goes hand-in-hand with Tolkien's drawings and Steve Harrington's quote. Yet, to follow the slimmer shape present from the drawings and the description in the book, it was redesigned to be a wyvern (serpe) in the second film, which is slimmer and has two less legs. Funny enough I think one of the reasons behind it was to find a middle term between the book and the drawings when it comes to legs / shape of the creature.
I do not know the easons why Smaug was depicted as having only two legs in the second and third Hobbit movies.
But if being faithful to the book is considered desirable in the Hobbit movies, changing Smaug from four legs to two legs should be considered an error. I think that it is absolutely certain that Tolkien always imagined that Smaug had four legs, and that the movies thus drastically changed Smaug's physical form from the book.
Dragons are often depicted as snake-like or lizard-like creatures.
Snake and lizards are quite closely related creatures. They resemble normal quadrupeds in many ways; both snakes and lizards have heads attached by necks to torsos that in turn are attached to tails.
The torso of a lizard begins with the shoulders where the neck and the front legs are attached and ends with the pelvis where the back legs and the tail are attached.
And that is also true for snakes. The torso of a snake begins with the shoulders where the neck and the front legs are attached and ends with the pelvis where the back legs and the tail are attached.
You say no, snakes don't have legs attached to their shoulders and pelvises. Actually some "primitive" snakes like pythons and boa constrictors have vestigial limbs remnants within their bodies, and even having spurs sticking out from their pelvic areas. The hearts, lungs, intestines, etc. of snakes are usually within their torsos and not their necks or tails.
In medieval English, "worm" meant a long, cylindrical animal, like a worm, a snake, or a dragon. And medieval depictions of dragons depict them with no legs, two legs, or four legs, with no wings or legs, with wings and no legs, with wings and two legs, with wings and four legs, etc.
And Tolkien certainly was familiar enough with medieval English to know that all dragons were "worms", regardless of how many or how few limbs they had.
Tolkien wrote a scene in The Hobbit where Smaug is described as "The Worm of Dread":
But that certainly would not inhibit Tolkien in describing or depicting Smaug with wings and legs.
In the Silmarillion the first dragon to appear during the First Age was Glaurung.
Names, epithets & titles
As the first and greatest of the dragons of Morgoth, Glaurung was sometimes referred to as the Great Worm,[3][4] the Worm of Morgoth,[5] the Great Worm of Angband[1] and the Gold-worm of Angband.[6] He bore the epithets the Golden,[7] and the Deceiver[source?] (the latter presumably referring to his dealings with the children of Húrin). He was also described as the Father of Dragons[7] and the first of the Urulóki,[8] though whether this implies he actually sired the rest of his kind is uncertain.
Glaurung was wingless, and the first flying and winged dragons did not appear until the Great Battle at the end of the First age.
Although Tolkien described Glaurung as wingless, and as a "worm", Tolkien depicted Glaurung as having four legs.
Here is a link to an image of Thror's Map from The Hobbit. It has a drawing of Smaug at the Lonely Mountain. But it also contains another drawing of a dragon, near an arrow pointing left (to the north in this map) and a legend "Far to the north are the Gray Mountains & the Withered Heath whence came the Great Worms." And there is an image of a great worm, wingless, but with four legs. http://tolkiengateway.net/wiki/File:J.R.R.Tolkien-_Thror%27s_map.jpg1
Here is a link to an image of "The Death of Smaug" by J.R.R. Tolkien, clearly showing Smaug flying, with two wings and four legs.
jmbpiano's comment to this answer also points out that Tolkien mentioned Smaug's "foreleg", a phrase which strong implies that Smaug had rear legs.
Here are links to other images of dragons by J.R.R. Tolkien, showing winged dragons with four legs. In short, J.R.R. Tolkien always drew dragons, both winged and wingless, as having four legs.
Some have suggested that Tolkien's picture "Conversation with Smaug" shows Smaug two legged. You will note that Smaug is coiled around a pile of treasure and that only his head, neck, tail, and the forward part of his torso are visible. The rear part of Smaug's torso, where his hind legs would be expected to be attached, is hidden behind a pile of gold, silver, and jewels.
A pair of legs is shown attached right in front of where the wings are attached. I would expect that most drawings of two legged dragons or wyverns would show the wings attached at the shoulders and the legs attached at the hips, like birds and bats and prehistoric flying reptiles. It would be very unusual to depict a wyvern with the hind legs turned into wings instead of the forelegs.
So I think that Smaug is depicted as a four legged dragon in "Conversation with Smaug", and the idea that he might be two legged there has never occurred to me before reading the answer where that is suggested today.
And you may remember a verse from the song in "an unexpected party":
The mountain smoked beneath the Moon
the Dwarves they heard the tramp of doom.
They left their hall to, dying, fall
beneath his feet, beneath the Moon.
Some of the Dwarves who sung that were witnesses. So apparently Smaug crushed many Dwarves to death beneath his mighty feet. Maybe Smaug had only two feet and held the front of his body far above the Dwarves on his forelegs while the rear of his body dragged on the ground. But Smaug's belly was soft and unarmored and could easily be pierced by axes, swords, and spears. So it seems likely that Smaug was standing high on four legs with his belly out of reach of any Dwarves trying to stab him with spears like Elaezar Maccabeus at the Battle of Beth-zechariah.
So if Smaug was depicted with four legs in the first Hobbit movie, it was because the filmmakers assumed that all dragons have wings and four legs, or else because they were familiar with Smaug with the book and knew that in the book Smaug had four legs.
Therefore, the special effects team deliberately changed the appearance of Smaug for the second and third Hobbit movies, for reasons discussed in other answers.
The purpose of my answer is not to discuss the decision making for the change in the Hobbit but to prove that - to the degree that someone thinks that the Hobbit movies should be faithful to the book - that decision was an error.
I would like to complete the other answers with quotes from the book Smaug: Unleashing the Dragon made by Weta Workshop:
SIX LIMBS OR FOUR?
For Peter, it is never too late to have a good idea. The brief for Smaug came in hints and clues: he was huge, he was incredibly old and maleficient. Red-gold, as Tolkien described. Winged. Fire-breathing. Hundreds of designs were done and a design for the Dragon was completed, built and glimpsed in the first film.
You might think we were done, but Peter decided that he should have two legs instead of four. Back to our drawing boards we went, but most happily as it turns out. In the design that emerged Smaug was stronger in every way, with wing 'hands' that became much more expressive, and a more aerodynamic and powerful silhouette.
John Howe, Concept Art Director
The next page in the book explains a bit more the changes in the design:
[...] When standing he was starting to look too much like a bulldog.
At some point, Peter asked about whether we should in fact be rethinking the anatomy entirely and consider exploring a creature that was much more snake-like, with only two legs. [...] Peter was looking for ways to involve [the wings] in the acting and use them to help convey emotion.
The excerpts quoted can be seen here (0:25)
It might also worth noting that they fixed the legs in the extended edition of the first movie:
Source: Dragons vs Wyverns: The Question of Smaug
So as explained in the other answers, it was not an error but a change in the design choice.
