With the recent release of the final trailer for Fantastic Beasts: The Crimes of Grindelwald, which led to the reveal of Nagini being a Maledictus, J.K. Rowling took to Twitter to answer some fan theories.
One big theory that Rowling herself thought she squashed down years ago, was that Nagini was the boa constrictor from The Reptile House at the London Zoo in the very first film, Harry Potter and the Sorcerer’s (Philosopher’s) Stone.
Noooo… I thought I’d shot that one down! The escaped boa constrictor in Philosopher’s Stone wasn’t Nagini. It was never Nagini. That’s an incorrect but very persistent fan theory! https://t.co/QEy89kwloA
Source: IGN.com JK Rowling Squashes Years Old Incorrect Nagini Fan Theory (Again)