Here is a list of movies that many people don't realize are comic book adaptations. I rank them on a combination of their popularity and the amount of people that don't seem to know they were originally comic books. IE most people wouldn't know that Ghost World was a comic book, but then again, most people wouldn't know that it was a movie lol making the fact they didn't know about the comic less surprising.
The film was based on Lowell Cunningham's The Men in Black comic book series, originally published by Aircel Comics.
based on the 1989 comic book of the same name by James O'Barr.
loosely based on the Marvel Comics character Blade, who first appeared in Tomb of Dracula #10
a 2003 superhero film adaptation loosely based on characters from the comic book limited series The League of Extraordinary Gentlemen by Alan Moore.
based on the 1998 comic series of the same name by Frank Miller.
American horror film based on the comic book miniseries of the same name.
the film is based on Vertigo Comics' Hellblazer comic book, with plot elements taken from the "Dangerous Habits" story arc (issues #41-46) and the "Original Sins" trade paperback. The character of John Constantine was introduced by noted comic book writer/creator Alan Moore while writing the Swamp Thing (comic book), first appearing there in June 1985.
Weird Science was a 1985 movie produced by Joel Silver, who acquired film rights to EC Comics in the early 1980s. The plot of the 1985 Weird Science film is an expansion and modernization of the basic premise in Al Feldstein's story "Made of the Future" in the fifth issue
I put then one last only because they are not original comic book characters. Sure Alien first met Predator in comic book form, but they were both based on movies.
