T.H. White's Once and Future King went through several revisions (including, unfortunately, the removal of the Madame Mim fight that showed up in the Disney movie). Only to say: such edits are not unprecedented; few things are the Bible (itself a heavily-edited book).
> including, unfortunately, the removal of the Madame Mim fight that showed up in the Disney movie
Where does this character come from? Searching for Madam Mim or Madame Mim seems to turn up nothing but Disney results. Searching wikipedia for Madam Mim does nothing but redirect to the cast of The Sword in the Stone.
Wickedpedia makes a claim similar to yours:
> Madam Mim appears in the original version of the novel, but not in the revised version featured in The Once and Future King
This appears to say that "the novel" refers to some other novel than The Once and Future King, but of course there is no primary Arthurian source in the form of a novel. I can't really understand it.
It's difficult to extend Wickedpedia the benefit of the doubt, because it goes on to say this:
> Kahl animated her [Madam Mim's] initial interaction with Arthur while Thomas oversaw her famous Wizards' Duel with Merlin. He is a witch Madam Mim who is more which than oversaw. He is also a tiger similar to Shere Khan from The Jungle Book, though he is not in a forest.
The character was in the initial version of the Sword in the Stone. The Once and Future King has a revised version without Madam Mim along with other modifications. The Once and Future King is not a novel but a collection of four previously published works. It is also missing the beautiful finale: The Book of Merlyn.
You can still buy the Sword in the Stone as a standalone novel, which will be the original version and includes the character. I bought an illustrated copy for my son.