For serious fans of the Harry Potter franchise, the Fantastic Beasts movies have been a welcome addition to the canon. They have filled in a lot of backstory for different characters and events only referenced in the original novels. However, one character who has become surprisingly important in the prequel films is Nagini, a character we knew only as a snake from the main series, but we learned was actually human in Fantastic Beasts: The Crimes of Grindelwald. While this revelation was a surprise to many, J.K. Rowling says it should not have been because the information was always there. According to the author...
These movies have given me the chance to tell a story that I knew all along about Nagini, who appears only as a snake in Potter. Now, there were always hints that she had been human. In her name — because the Naga are, in mythology, a race of snake beings. So the name was an allusion to the fact that she may herself once have been human.
The Naga is a mythological being that is actually half human and half cobra, not one that transforms between the two states as the character does in Fantastic Beasts, but the connection is still obvious. Nagini would be the feminine version of the word, telling us the gender of the snake that stayed by the side of Lord Voldemort.
J.K. Rowling has built a reputation over the past few years of dropping some interesting information about various characters in Harry Potter that is never stated in the text, but according to her has always been part of the official back story. This particular item, which was reveled in one of the Blu-ray extras attached to Fantastic Beasts: The Crimes of Grindelwald (via the Insider) seems to be a little more on the level then some of the revelations we've been given.
The connection certainly makes sense. Anybody who was familiar with the etymology of the name Nagini could have potentially made the connection on their own, some probably did. While it doesn't guarantee that J.K. Rowling had always seen Nagini as formerly human, she even says herself that the allusion was meant to imply she "may" have been human, it certainly left the door open to the idea if the need for such a plot point were ever to surface.
Of course, now that we have met the human version of the character, we have a lot more questions. Nagini appears to be one of the good guys following the events of The Crimes of Grindelwald, certainly more so than some others who have aligned themselves with Johnny Depp's dark wizard character. However, we know where this character will ultimately end up, so quite a bit will be changing over the course of the planned three remaining movies.