That is when Warner announced that Ms. Rowling had agreed to adapt for the big screen her “Fantastic Beasts and Where to Find Them,” a 2001 book billed as one of Harry Potter’s Hogwarts textbooks. Three megamovies are planned. The main character will be a “magizoologist” named Newt Scamander. The stories, neither prequels or sequels, will start in New York about seven decades before the arrival of Mr. Potter and his pals.