It's later revealed that Nagini was never an ordinary magical snake from start, as she was once in fact, a human (possibly a Witch) who became a snake as she is now as result of a blood curse and trapped in that form for the rest of her life.
J.K. Rowling recently gave more insight into one of the most controversial characters in the Wizarding World: Nagini, who was known only as Voldemort's snake in "Harry Potter," but appears as a woman in the prequel series, "Fantastic Beasts."
Voldemort has no wish to know anything about anyone outside of himself. It simply wasn't important to him as it didn't give him any power or knowledge. However, it may have helped him use her as a Horcrux possibly because Nagini being formerly human may have helped her be strong enough to become a Horcrux.
Lord Voldemort and Nagini had a special relationship
As a Parselmouth, (i.e. able to talk to snakes) Voldemort could communicate with her, and the pair often sent each other messages, even suggesting a telepathic connection at times.
Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanently. Though it is unknown if she became evil as a result of that, or was turned evil as a result of Voldemort turning her into a Horcrux. As a snake, Nagini was completely loyal to Lord Voldemort.
Dumbledore does not use any language to make one think that Nagini is anything more than a snake. But this could be despite his knowledge of Nagini's past because he has to factor his grand plans into everything he tells (or doesn't tell) Harry.
When Harry and Hermione are on the first floor of Bathilda's house, Nagini (inside the corpse of Bathilda) tells Harry to "Come!" from the next room in Parseltongue. In reaction, Hermione jumps and clutches Harry's arm, and the two of them obey the command.
By 1994, Nagini was seemingly trapped in snake form and belonged to Lord Voldemort, with whom she had a special bond largely due to becoming a Horcrux after her master had killed Bertha Jorkins in 1994.
Deathly HallowsEdit
Nagini, Voldemort's snake, then eats Professor Burbage's dead body. A story about her "resignation" from Hogwarts appears in the Prophet, but members of the Order of the Phoenix discount it, as they are unable to locate her anywhere after that.
Nagini is loyal to Voldemort because he can speak with snakes. As a Maledictus, there probably isn't much time after the events in The Crimes of Grindelwald before Nagini falls to her blood curse, becoming a snake forever.
There was a rumour that Nagini was the snake Harry released from the zoo in Harry Potter and the Philosopher's Stone. This was proven false, as Nagini is some type of viper while the snake in the zoo was a boa constrictor. Furthermore, the snake was given a male voice in the film, while Nagini is female.
Voldemort orders Nagini to kill Snape, and the great snake bites Snape's neck, mortally wounding him.
It wasn't just inanimate objects that could become Horcruxes. Lord Voldemort's beloved snake companion, Nagini, was also transformed into one, and Dumbledore seemed to know it, ominously predicting that a day would come where Voldemort would 'fear for his snake'.
He uses his snake, Nagini, to kill Snape rather than any number of spells he knows. This allows Snape to live long enough to help Harry defeat him.
Harry can no longer speak it
Harry being an accidental Horcrux meant he was bound to Voldemort in so many ways, just like Voldemort was bound to serpents. Not only could Harry speak the language of the snake, but could see through the eyes of Nagini, another of Voldemort's Horcruxes, as it turned out.
He always seemed like he truly loved Nagini. He must have felt something for Nagini or he wouldn't have given her a part of his soul.
In the movies, Nagini was the one who killed Snape in the boathouse. The Maledictus meets her end when Neville Longbottom finds Gryffindor's sword and, in an attempt to destroy the final Horcrux, cuts off her head.
But when Ron and Hermione are about to die by Nagini, Ron casts the killing curse as a last resort and it simply rebounds off Nagini as it was nothing.
She was dieting and didn't want greasy food.
Harry Potter and the Deathly Hallows
In this book, Rowling reveals that Bellatrix is the guardian of Helga Hufflepuff's cup (though she is unaware that it is a Horcrux), which Voldemort has entrusted the Lestranges to keep in their Gringotts vault.
He was given a Blood-Replenishing Potion, as he continued to bleed profusely any time his bandages were removed, suggesting that Nagini's venom somehow prevented or delayed blood clotting.
Therefore, when Voldemort used the killing curse against Harry in Deathly Hallows - Part 1, he was actually killing part of his own soul, and Harry was able to survive. And after Neville killed Nagini, all Horcruxes were destroyed.
Voldemort, weakened by his long stint as "less than the meanest ghost' 69 insists on making one more Horcrux. And unlike with Harry, Voldemort did say the spell when he killed Bertha Jorkins to make Nagini a Horcrux.
As an unqualified wizard who didn't finish his education, Hagrid was not technically allowed to perform magic.
It is very possible that J. K. Rowling did say this, but that she was referring to the snake actors in the movies, not the characters. This is NOT true. The quote was started as a joke on tumblr. Besides, Nagini is a viper (a venomous snake), and the snake Harry set free was a boa (a constrictor, not a venomous snake).