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.
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.
After Voldemort was gone, Nagini decided to follow her dreams. She went to Brazil to join other snakes in the Amazon rain forest. While there, her adventures with Voldemort inspired her to write a song in Parseltongue called "I Thought You Were Pretty Cool But Then You Couldn't Even Defeat a Baby.
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.
The only "emotion" Voldemort felt when Nagini died was fear because she was the second to last piece of his soul. If Nagini wasn't a horcrux, he wouldn't even have kept her around. When Voldemort said “I regret it” after killing Snape, what was he feeling?
In 1994, Death Eater Peter Pettigrew milked the venom from Nagini's fangs and used it, along with unicorn blood, to concoct a dark potion, which allowed Lord Voldemort's shattered soul to regain a rudimentary, but physical form.
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.
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'.
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.
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.
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.
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.
But now we know Nagini was no simple supervillainous pet, but a human woman cursed with the “Maledictus” blood curse, which, according to Rowling, is a female-only inherited condition which grants the affected individual the ability to transform into an animal at will, until one day, the transformation becomes ...
Voldemort orders Nagini to kill Snape, and the great snake bites Snape's neck, mortally wounding him.
As a result, she is kidnapped by Death Eaters and summarily executed by Voldemort. Nagini, Voldemort's snake, then eats Professor Burbage's dead body.
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.
Rowling previously revealed that Nagini is a Maledictus, which means she carries a blood curse that will doom her to transform permanently into a beast. In a film extra called "Credence, Nagini and the Circus Arcanus," available on the "Crimes of Grindelwald" Blu-ray, Rowling said she "knew all along about Nagini."
The wand will not perform its best for Voldemort until Snape is dead. So saying, Voldemort waves his pet snake, Nagini, in her nest of protective spells, into Snape.
Troubled, Lord Voldemort spoke to Nagini in Parseltongue, telling her that their secret had been discovered and warning her to never leave his sight, before Disapparating to a cave to seek out the locket of Salazar Slytherin.
He doesn't trust Snape as Dumbledore does, but he believes that he can detect any disloyalty. Moreover, Voldemort regards Snape as highly intelligent and therefore ' as a true Slytherin ' a person whose top priority is to take care of himself.
The ability to speak Parseltongue is usually inborn; a wizard is born being able to speak Parseltongue, and those who are not born with the ability cannot normally learn the language (although, according to the author, Dumbledore may have learned to understand it, and in Harry Potter and the Deathly Hallows, Ron learnt ...
Once he learned that his horcruxes were being hunted, he knew Nagini was at risk. He couldn't know how his enemies had been able to find his other horcruxes, but he had to suspect at the least the possibility that it was by some magical means unknown to him, perhaps devised by Dumbledore.
Lord Voldemort callously fed Muggle Studies teacher Charity Burbage to Nagini after performing the Killing Curse.
He finds out for sure immediately after Nagini's attack on Arthur Weasley. This night is significant in many ways. While Harry sleeps, he enters Nagini's mind.