JK Rowling may really have set-up Nagini being a human from the start and the proof is in Book 6, Half-blood Prince. The "Secrets of Dumbledore" might be setting up a plot twist on Voldemort's origin involving her.
Harry Potter
The Dangerously Stupid Animus against JK Rowling
In Defense of JK Rowling and in Opposition to Cultish forms of Trans Activism