When you stop and take a moment to think about dragons, chances are you have a very specific image in your mind of what this mythological creature looks and acts like, depending on whatever your formative dragon experience was.
So you might find it weird that a dragon can actually be a lot of different things to different people. A lot of the differences in dragon representations are cultural; for example, Westerners trend more towards a dragons-are-evil mythos, while others, especially the Chinese, revere dragons as bringers of luck. Dragons have also changed over time; we've somewhat overcome our stereotypical dragon sensibilities in favor of the more modern notion of seeing dragons as 'people,' or at least assigning some sort of human or uber-human characteristics to them.
Below is a handy guide to the panoply of dragons in movies that might, perhaps help you learn how your ideas of what a dragon is came to be, and whether or not those ideas are terribly wrong. The entries are listed roughly in chronological order, so you can see the evolution of dragon representations over time.
Maleficent in Sleeping Beauty (1959)
No representation of dragons in movies is more traditionally Western than that of Maleficent in Sleeping Beauty. If you went back a few centuries and showed this movie to a bunch of townspeople… well, first of all they'd flip out over seeing moving pictures… and then they'd probably torture you and burn you at the stake for performing witchcraft. After that they'd all sit down together and say, "Yeah, looks like a dragon to me!"
A lot of Western myths use dragons as symbols of the devil. There are no redeeming factors, nothing besides true, unadulterated evil to this terrifying creature. The fact that this dragon actually is a wicked witch just tops off the perfect image of everything we've come to know about good and evil.
Smaug in The Hobbit (1977)
This is another dragon that's pretty one-dimensional; however, we're starting to get a bit more into the mythos of dragons with this character. We can't say for sure if Tolkien invented the idea of a treasure-hoarding beast living forever in a cave, but he invented pretty much everything else we know of as fantasy, so it's a fair bet.
This dragon eats people, destroys towns, is greedy, and thinks only of himself. At least Maleficent only put people to sleep and blows a bit of fire now and then. This is possibly the worst-behaved dragon on our list.
Elliot in Pete's Dragon (1977)
We're not sure exactly what to make of this. The same year that brought us Smaug also brought us Elliot, the misbehaving dragon that may or may not have been a figure of Pete's imagination induced by too many beatings or excessive drug use. Of course, by the end everyone can see Elliot, so maybe the drugs were in the water?
Maine has always been a kind of 'live and let live' place, so it's no surprise that they set a movie with a heart-of-gold dragon in a town there. Still, although Elliot cares for Pete very much, he's got some characteristics that wouldn't be tolerated in an upstanding human citizen. He loves pranks such as pulling out a lady's slip and getting egg all over people's faces. And that's the true essence of dragons - whether they're good or bad, they're never entirely human, and don't necessarily play by our societal rules.
Falcor in The Neverending Story (1984)
This is the first movie on our list based roughly on Eastern folklore. Falcor is a luckdragon, which you might be surprised to find out isn't a real actual thing; however, especially in Chinese culture, dragons are considered lucky; so that's basically where the inspiration behind Falcor came from.
His ability to talk is an important distinction, because talking makes a mythological creature much more human than not-talking. It's easy to write something off as purely good or purely evil based on actions; when that same thing talks, the whole equation gets a lot more complicated. Falcor himself is complex. He seems to have powers that he can use for good, and yet he allows the human characters to make their own mistakes. Either he's very smart, or he's kind of a dick. We'll say for the sake of argument that he's mostly good, with a bit of unpredictability thrown in.
Draco in Dragonheart (1996)
Here's another dragon who has the talking thing down. We first see him trying to take down the hero of the movie, so we figure right away that he's not a very nice guy.
What we learn later is that things are much more complicated than they seem. The main evil-uber-bad dude has half the dragon's heart (hence, the title of the movie!) and the evil guy can't be killed unless the dragon is killed. And did we mention that the dragon is also the last of his kind? Oh, the moral quandries! Who knew the mid-90's were so complex?
In the end (spoiler alert!) the dragon sacrifices himself to save the people from the evil king. So, he turns out 'good' in the end - although we have to ask if sacrificing yourself for members of another species, most of who hate your guts, really qualifies.
Mushu in Mulan (1998)
Mushu is another dragon who is roughly styled after Chinese dragon mythology, although, as usual, the moviemakers have taken liberties.
Again, here is a dragon that tends to get in trouble. Dragons are pretty much always troublemakers, have you noticed this? We suppose they wouldn't be very interesting if they just sat around drinking tea all day, but it does limit a dragon actor's repertoire.
Luckily, most of Mushu's trouble-making does seem to turn out okay in the end, so we guess that he's on the good side.
Dragon in Shrek (2001)
We're going to go ahead and say that this is a step back for dragon kind. We're not sure exactly what they're trying to say with this dragon character.
First of all, she's a killer who has already killed several knights trying to rescue Fiona. So bad, right? Well, it turns out she's not really evil, just really lonely. Um, we guess we can get behind that. Then she pretty much has her way with Donkey, which we don't think is really the right message to be sending anyone.
After that she escapes her bonds, what does she do? She mopes by a lake and waits for Donkey to find her. He, by this time, has somehow fallen in love with this creature who breathes fire and is about 100 times his size. Why? We don't see them conversing. She doesn't speak. She's more like a pet. And then she roasts a bunch of guards. So, it's hard to call her truly evil, but she is all kinds of weird.
Haku in Spirited Away (2001)
Haku starts out as a spirit-boy who works to save the human girl who has stumbled into the spirit realm to save her parents, who've been turned into pigs. Yep.
He's not exactly a dragon, per-se; he's a river spirit who has lost his way. In his confusion, he forgets who he really is and helps a witch do her bidding. He ends up doing some things that aren't very nice, but this story is very true to the nature of Japanese folklore; no one character is either all good or all bad. Things are just the way they are. And everything turns out okay in the end, so we'll stick Haku on the good side.
Dragons in Harry Potter (2005)
These are by far the most animalistic and least human dragons on the list. They don't talk or in any way communicate with humans.
The thing is, if dragons existed on Earth, we'd consider them animals. You could argue that a dragon-like creature could have evolved a higher intelligence than other creatures, but there's no saying that it would be the kind of intelligence that we would understand or even recognize. So it would be alien to us. The dragons in the Harry Potter movies are just that - animals who live, eat, fight, breed, and die. We see through Hagrid that even the strangest of creatures should be respected, but the dragon characters are clearly just what they look like - glorified dinosaurs. And you can't really assign good and evil to animals. Although their status as wizard-killers, and their attempt to chomp the lovable Harry Potter, don't put them in a very good place in our eyes.