I also like King_Erlian's idea (MN shows her real origins while Mr Beaver is only expressing an opinion) as a good in-world explanation. Perhaps it's not so much that Mr Beaver's being pompous but that he's repeating a legend or story he heard as a young beaver.

(I wonder if Lewis had had the opportunity to edit the Chronicles after they were all finished he might not have addressed the two versions of her origins).

As others have noted, Narnia is full of mythological creatures; I think Lewis included them as a nod to his classical education as well as a way to make Narnia a more fantastical place. I don't think he believed in any of the creatures as such.

Perhaps one reason he chose to include Lilith and Jinn was to add Jewish and Arabian elements to the Greek, Roman, and (in the old US versions) Norse mythologies already in the story.

first, to answer the original questions in this topic:

1. No, I don't think that Lewis believed in the existence of Lilith or the Jinn - but, he always appreciated a good story. And Dernhelm_of_Rohan hit it right on the head when they stated that you might as well ask if Lewis believed in talking trees.

2. I think he chose Lilith to show that Jadis was a true monster - and the Jinn to show her magical side.

3. No, I really don't know any more than that.

Now, to address the seemingly different accounts of her origin found in LWW and MN - I've been pondering this one for years. I've finally come to the conclusion that the accounts don't conflict. And I don't think that the Beavers were incorrect.

Consider this - Lilith was banished (or escaped) to Charn which is the homeworld of the Jinn. Remember in PC after the Pevensies were summoned to Narnia by Susan's horn? I think it was Edmund who says something to the effect of their being called to Narnia just like the stories of the genies being called forth by rubbing a lamp - but no one ever thinks about where the genie is coming from. Maybe it was Charn??

Food for thought.

Questions for discussion:

1) Do you think Lewis believed in their existence, since he put them in his book?

I think it was a good point that Warrior4Jesus, used saying that, myths are becoming real in Narnia.

Like many of the other myths, it hard to say exactly why those in particular, but overall, I think it was a good choice. It really showed that Jadis was not the only really evil one in her family.

Also, I think she kind of had to be at leasted ''related'' to humans to look soewhat like them. This is just what Lewis chose.