Nagini Is A Real Person in Harry Potter
This is one truth that has enraged many fans due to its unsettling implications. J.K. Rowling declared it canon in Fantastic Beasts 2 that Nagini was a person all along. She was a "Maledictus," which is a human with a blood curse that causes them to transform into an animal. It's similar to an Animagi, only they didn't choose to transform. Nagini would eventually become permanently trapped in that shape, which happened when she was with Voldemort. This came perceived as a significant retcon, and it made Voldemort's bond with his serpent servant much more despised in the eyes of fans.