Well, ask and ye shall receive. Regular reader Ian M. Slater wrote in about that history:
The substitution of Fenris Ulf for Maugrim is an interesting example of a
highly specific, externally verifiable, allusion, replacing Lewis' own
invention. (There is apparently no source for "Maugrim," although I was once
under the impression that a version of "Reynard the Fox" had something
There were a few others, plus some other changes, all of which are
documented in (the successive editions of) Paul F. Ford's "Companion to
Narnia" (1980 and following). The 1994 edition decried the announced change
from the American to British texts, and he continued to protest in the new
(2005) Fifth Edition.
Ford makes a good case that (barring some actual errors) the revisions were
made by Lewis in galleys, and were deliberate improvements, in some cases
responses to observations by readers; and worth preserving.
In connection with Fenris, he points out that Norse material was injected
into LWW at another point -- a substitution of "the World Ash Tree" for "the
fire stones of the sacred hill" in one of the White Witch's comments. The Norse
element in the whole book seems to have been made a little more overt, as if to
counterweight the existing, very obvious, classical material.
Not everyone agrees that any or all of the changes in the American editions
were improvements. See Ford's note to his "World Ash Tree" entry for a defense
of the original reading there.
I think that they were; or most of them. And it is not as if Lewis was given
to second-thoughts, and frequent tampering!
My opinion in favor of Fenris and the World Ash is probably shaped by the
fact that I read the American text while in sixth or seventh grade, and
recognized and enjoyed the allusions. I recall being surprised a bit later that
they were not present in the Puffin paperback, which was otherwise far and away
the most attractive edition I had seen, and checking against a library copy to
be sure that I had remembered correctly.
And it wasn't as if I was giving Lewis the sort of close reading I did with
the Ace and Ballantine texts of "Lord of the Rings." Ford points out at
least one textual corruption in the Puffin "The Last Battle" which changes
the whole impression of a passage, and which I never noticed. (Lucy's
typically compassionate appeal for the "poor, stupid, dwarfs" became just
"stupid dwarfs," which is much less effective as a plea for mercy, if not
Just to add one small note, "Maugrim" might not have a specific source, but it seems to me an obvious conflation of the germanic words "maw" and "grim." Maw might be a bit archaic nowadays; in the Anglo-Saxon leechbooks "maw" seems to refer to the stomach more than the mouth, though I take it to mean the digestive tract from mouth to stomach. The allusion, then, is more philological than mythological.