Maybe when he was writing The Lion, the Witch and the Wardrobe, C. S. Lewis imagined Narnian dragons being neutral or good and when he was writing The Voyage of the Dawn Treader, he realized that having them be dangerous would make the story more exciting (with everyone scared of Eustace before they find out his identity.) 

I'm not sure about this theory because I feel like C. S. Lewis doesn't typically make the fantasy creatures traditionally considered evil good in Narnia. Then again, he does fauns, satyrs and centaurs were evil, or at least amoral, in Greco-Roman mythology. But Lewis's fauns seem to have been influenced more by Edmund Spencer than mythology. As far as I can tell, it's either the first theory or Aslan didn't restore all the statues, which feels wrong to me, reading the book.

It's get kind of interesting. According to the timeline, there was a time where after some Calormenes settle in Telmar, Aslan turned them into dumb beasts for their wicked behavior. It would be safe to say that among the dumb beasts was a dragon.

In the case of Eustace in Voyage of the Dawn Treader, he was turned into a dragon after he was  hoarding all the treasure he's found. Dragons have known to be hoarders of treasures. Same thing may have happened to Lord Octasien.

Posted by: @the-mad-poet

Right now the two best explanations seem to be that dragons are typically evil creatures, or... Lewis just forgot.

Probably the latter, because--well, it happens.  So far as I recall, he didn't mention the winged horse again, either; perhaps he substituted unicorns instead?

However, it's an interesting thing on which to speculate.  We do tend to imagine that all the ossified creatures were good; do we have evidence for that?  Not to claim that the dragon was inevitably evil, or anything like that.  Simply put, anyone who displeased the WW was at risk of being turned into a lawn ornament.  She didn't strike me as someone who was very patient or forgiving of failure.  Maybe Maugrim was lucky to have been killed by Peter after all? 

Posted by: @aileth

  We do tend to imagine that all the ossified creatures were good; do we have evidence for that?  Not to claim that the dragon was inevitably evil, or anything like that.  Simply put, anyone who displeased the WW was at risk of being turned into a lawn ornament.

True enough, but I feel like it makes What Happened About the Statues less cool if some aren't returned to normal.  

Posted by: @aileth

Posted by: @the-mad-poet

Right now the two best explanations seem to be that dragons are typically evil creatures, or... Lewis just forgot.

Probably the latter, because--well, it happens.  So far as I recall, he didn't mention the winged horse again, either; perhaps he substituted unicorns instead?

I suspect "Lewis just forgot" really is the most likely answer here. There are plenty of other instances in the Chronicles where that obviously happened — particularly the huge disparities between what we're told of the White Witch's origins in LWW and what we see of her as Jadis in MN, but even in minor details like Reepicheep in VDT sharing the song that a Dryad sang over him when he was a baby... at a time when, according to the previous book, the spirits of the trees were asleep and weren't awakened until Aslan returned!! (I never picked up on that inconsistency myself until I read it in a commentary somewhere.)

I have talked about dragons in my Sunday School class, now that we're going through Voyage of the Dawn Treader. Creation scientists have said that dragons may have been referred to dinosaurs. (Maybe there are talking dinosaurs in Narnia, but I think that could be another discussion).

I guess Occam's Razor really applies here. Given that Lewis planned on revising the Chronicles shortly before his death, it could be assumed that the dragon issue might have been on the list of inconsistencies to be dealt with. 

  As for the "dragon got left out" theory..  I'm inclined to agree with Col Klink, that would make the statue scene a good deal less cool. 

  The more I read Narnia, the more I realize there are going to be things like this that have no explanation... outside of the author's mistake. (I guess every Middle Earth fan can probably relate to this with the 'eagle problem.'). The occasional error kind of sucks, but the overall greatness of the stories tends to make up for it!  

I would think Narnia had some good dragons as well as evil ones.  Eustace became a dragon, and the experience wasn’t entirely negative for him. In the fairy tales the dragons horde treasures and breathe fire, but I can’t imagine that creatures that Aslan made would all be evil.  Eustace did not want the treasures and his feeling and attitude towards the other people had actually improved after he became a dragon.  I have also wondered about wolves in Narnia.  Were they all like Maugrim, or were some of them good creatures like the talking lions which were true beasts like Aslan? It seems more likely that many of the talking animals (even wolves and lions) were good creatures devoted to Aslan.

Technically, if Edmund just "took" the creature to be a dragon, there's a chance it was really something else. But what? 

Posted by: @col-klink

Technically, if Edmund just "took" the creature to be a dragon, there's a chance it was really something else. But what? 

A wyvern? A basilisk? Not sure, but the number of large slithery critters which could be conceivably mistaken for a dragon is sadly limited...

That said, one wild theory which just occurred to me is that perhaps the creature is a Sea Serpent the Witch took a dislikes to. She might have petrified it somewhere on the coast and had her minions drag it back to the castle. Aslan probably wouldn't want to restore it, as it would either die on the spot (assuming they breathe water) or just be trapped on land ( and also useless as an ally in the battle), so it doesn't figure in that scene. 

There, that's TOTALLY what Lewis was thinking.