Does anyone still read the children’s book, Little Lord Fauntleroy? From the manner in which the name of the title character is used as an epithet, one might forget that the original depiction was positive.
A plot device rather than a character, but the Protocols of the Elders of Zion came from some crappy French novel.
No shit, Sherlock.
I nominate Topsy, Trilby, and Fedora.
BTW, I disagree with the premise of the OP re the claimed obscurity of Walter Mitty. I have in fact read the original short story, plus there is a rather famous film inspired by it. One of Danny Kaye’s best, in fact. I think that a sizeable percentage of people will have seen the movie, or at least clips of it, and still quite a few will have seen the original story.
Big Brother, from 1984. From ‘Big Brother’ the TV series to phrases such as ‘Big Brother database’, this has become even more of an everyday phrase than anything previously mentioned.
Just a quick “Me, too” endorsement for this book. It’s an outstandingly researched and fascinating look at Bara. I recommend it highly.
But note that the film is very different from the short story.
I disagree that the short story “The Secret Life of Walter Mitty” is still as well known as the titular character.
On gun-oriented discussion boards there is an endless stream of “Which gun for ____” threads. When _____ is something too fanciful or bizarre, I frequently reply with
Nobody ever gets it.
Ta-pocketa-pocketa, all.
Harlequin, Scaramouche and Pantalone were standard characters in the improvised Commedia dell’Arte dating back to 16th century Italy. Pantalone was identified by his long-tight-fitting trousers, eventually leading to the word “pants”.
Doctor Doolittle? I’ve never seen the movie/read the book yet I recognize the reference.
Does it count if a derived work is well-known? Most people have seen the Disney versions of Mary Poppins and Peter Pan, but very few will have read the books in which the characters were introduced.
It’s worth mentioning that “A Fool There Was” is a recurring line in the poem “The Vampire” by Rudyard Kipling, and if one took one’s cue for the script from that poem, it’s not at al surprising that you’d have a character called “The Vampire”. The contribution of this film would be the shortening of it to “The Vamp”.
http://www.vampgirl.com/poetry-kipling.html
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The original Mentor from The Odyssey and Greek mythology. It’s well known, of course, but I’d bet most people don’t mentally associate “mentorship” with the character.
Echidnas (spiny anteaters) and arachnids are propbably better known than the mythological characters they’re named after, Echidna and Arachne. Same for narcissism and Narcissus, hermaphroditism and Hermaphroditus, etc.
Also, calliope and Calliope, and panic and Pan. 
How about “Jeckyll & Hyde”? How many people have actually read the original story - compared to the amount of folks who are more familiar with the Tweety Bird parody?
Last week I was reading an article about the novel Lolita (it’s the 50th anniversary of its American publication) and the writer pointed out that while everyone’s heard of Lolita, very few people know anything about the actual character. Including her real name, which was Dolores.
On that note, I imagine it’s only a matter of time before “Kill the Wabbit!” becomes more well known as the name of Ride of the Valkyries than the actual title.
Dorian Grey