Neville Longbottom could have been the chosen one. As Dumbledore explained to Harry Potter, the prophecy said that the dark lord would Mark the child as his enemy. Only in the process of marking the child would Voldemort confirm which of the two children, Harry Potter or Neville Longbottom, would become the chosen one. When Voldemort tried to kill Harry Potter, he left a physical Mark in the form of a scar on his forehead. By doing that he made Harry Potter the chosen one over Neville Longbottom. He imparted on Harry Potter a piece of his soul, accidentally making Harry a horcrux, and he imbued Harry with powers he knew nothing about, just like the prophecy said.