Superman
Batman
Mickey Mouse (depending on what you mean by a “person.”)
Tarzan
Peter Pan
Dracula
Frankenstein (along with his monster)
Robin Hood
Hamlet
The Invisible Man
Dr. Henry Jekyll
Marco Polo and the entire kingdom of Camelot 
The Hunchback of Notre Dame
When I visited the famous cathedral, I kept peeking into nooks and corridors, hoping to see places where he might have bedded down or peered down at the populace. I hear a lot of other folks do this, too.
Marco Polo did exist, he was one of the first really famous bullshit artists. 
I was going to say Merlin, but now someone will probably point out I’m wrong.
William Ashbless, poet, cook and author.
John Worphin
John Big Boote ( I don’t know how to do the thingee over the E)
John Ya-Ya
John Small Berries
There’s pretty good evidence for some of them - notably Tristan (or Tristram) son of King Mark of Cornwall. Arthur has been addressed in this thread a couple of times. Merlin’s iffy. Ghanima addressed your misconception on Marco Polo.
According to Howard Shenkman’s Legends, Lies, and Cherished Myths of American History, John Henry did exist. He gives cites.
In fact, Shenkman devotes an entire section of his book to people who did exist and people who didn’t. Surprisingly, Johnny Appleseed existed. So did Uncle Sam, actually San Wilson, and he was one of Johnny Appleseed’s classmates. Again, see Shenckman for cites. So did Mrs. O’Leary and her cow, although it’s not known if they had anything to do with the Chicago fire.
Lancelot and Merlin have been treated above. Unlikely, both of 'em.
New York Mets pitching prospect Siddartha “Sidd” Finch.
Daniel Boone :rolleyes:
Daniel Boone was a fictional character loosely base on Davey Crockett.
People around here get them mixed up.
So, who is it that has said they believe these people exist? :dubious:
Marco Polo almost certainly existed, from what I’ve heard, although there’s a book out that doubts his journeys.
But Sir John Mandeville didn’t. At best the name is a nom de plume of a writer. At worst the guy made up his narrative from existing travel books. But the editor of the Penguin edition of his Travels seems to think that there may be real and original material in the work, and footnotes bits of Chinese trivia to bolster his case.
Penelope Ashe, the author of the book Naked Came the Stranger, didn’t exist. It was revealed a couple of years after the book’s release that the name was simply made up and the book was written by a group of authors, each doing a chapter. The ruse was so well remembered that about a decade ago a group of Florida-based authors (Dave Barry and Carl Hiassen among them) could pull a similar stunt and call the result Naked Came the Manatee, without having to explain the title.
For that matter, there are lots of “house names” for books that don’t have a single real person behind them. “Victor Appleton” and his nominal grandson, “Victor Appleton III” (authors of Tom Swift and Tom Swift, Jy. books), “Kenneth Robeson” (author of the Doc Savage stories. Most, but not all, were by Lester Dent). I have a few back home that I can’t recall. You can find lots of them listed in The Encyclopedia of Science Fiction and The Encyclopedia of Fantasy.
John Henry was real? That is a surprise.
Johnny Appleseed I knew about.
As I said, Merlin is “iffy” at best and Lancelot is a literary character.
Hillbilly Queen, I’m afraid Daniel Boone did exist, about 50 years before Davy Crockett. Back when “Western Frontier” meant “Kentucky”. Here’s some info.
There’s also a rather lengthy Straight Dope column about Johnny Appleseed. Weird, weird guy.
I hope Daniel Boone existed, otherwise I think he’d be the first fictional character to serve in the US Congress.
“It’s not my goddamn planet, monkey-boy!”
I think most of them have been fictional.
However it was Crockett who was in Congress.
Boone was in the Virginia Assembly FYI. Link to Boone timeline.
Sun Tzu is widely assumed to have been an amalgamation of several ancient Chinese warriors.
I almost forgot about the legendary (literally) Colonel Toon, North Vietnamese fighter ace of the Vietnam War…apparently several North Vietnamese pilots were assumed to be the same guy.
Though Sam Wilson did exist, it’s unclear if he had anything to do with Uncle Sam. The first mention of the connection was by a relative years after the people who could have confirmed or denied it had died.
If pen names count, there are many: Ellery Queen (which also was used as a house name; in addition to Dannay and Lee, Theodore Sturgeon and Avram Davidson, among others, wrote as “Ellery Queen”), Saki, George Eliot, James Tiptree, Jr., Mark Twain, and Don A. Stuart, to name a few.
Alt+0233 if you’re on a Windows box.
CalMeacham beat me to John Henry.