This was an ability the Circus Arcanus took extreme advantage of. Ultimately, however, Nagini's Maledictus blood curse took over, which forced her to stay in her snake form permanently.
The second film in the series, written by Harry Potter architect, J.K. Rowling, revealed in a trailer that fans had been on the right trail. Nagini, it turns out, was actually a Maledictus—a witch with a blood curse that turns her permanently into a beast (in her case, a giant snake).
Voldemort possesses Nagini because she's immortal.
YouTuber Seamus Gorman points out that Nagini's Maledictus curse could make her immortal, and we all know that Voldemort's thirst for power would have loved that.
Though it may later be revealed that the Dark Lord did indeed know that Nagini was a Maledictus, there's currently no indication that he (or anyone else) ever thought that she was anything other than a snake who was made into one of the Dark Lord's Horcruxes.
Nagini's lifespan was extended by being a horcrux. Nagini's lifespan was extended by other magic. Nagini spent some time in suspended animation and did not age. (perhaps while Voldemort was “dead”)
Nagini seemed integral to Lord Voldemort's survival before he could be restored to his body, and we learnt, quite grotesquely, that Voldemort even tasked Wormtail to 'milk' Nagini so he could live off her venom to keep his strength up.
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.
Unless one or the other has a significantly smaller role in the films than we are led to believe, Dumbledore and Nagini will get to know each other. From the very little we see of Nagini, it looks like she's at least potentially on the same side as Dumbledore.
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. Burbage is later replaced as Muggle Studies teacher by Alecto Carrow.
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'.
In the belief that the Elder Wand will grant its allegiance only to one who has slain its previous owner, Lord Voldemort has his pet snake, Nagini, kill Severus Snape.
In order to get her venom, Pettigrew had to milk Nagini for it. However, milking a snake is done via the fangs by having them bite a latex cover over a glass jar, then stimulating the venom sacks by either manually massaging them or using electricity to make the muscles contract.
In Harry Potter and the Deathly Hallows, Voldemort thinks that killing Snape will give him full control of the Elder Wand. 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.
At the end of the last film, Grindelwald told Credence he was really Aurelius Dumbledore – the previously unheard of sibling of Albus. However, that wasn't entirely true. While Credence is a Dumbledore, he isn't the brother of Albus, Aberforth and Ariana. In fact, he's Aberforth's secret son!
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.
So it's possible that Nagini met Tom Riddle in his early years of rising to power but by the time she found him in Albania she had been cursed to become a snake entirely, and losing the ability to turn back into a human.
Charity Burbage floats helplessly above a dinner table, as if paralysed. She begs Snape to help her. Voldemort point his wand at her, and a green light erupts out of its end. The light hits Charity, and she falls down on to the table dead, her eyes open and her face splattered with droplets of blood.
The Deathly Hallows Part I begins with a whole bunch of double-crossing and betrayals. Snape betrays the Order of the Phoenix. A guy from the Ministry of Magic becomes a Death Eater, which is probably how the Ministry ends up falling. And that's just the beginning of the movie.
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.
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."
Sorcerer's Stone - he speaks to the snake in the cage at the zoo. Chamber of Secrets - he tells Draco's conjured snake not to attack another student. Deathly Hollows - he speaks with Bathilda Bagshot (who is actually Nagini).
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 ...
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).
Nagini the Snake
Nagini was the last horcrux to be destroyed and also the last to be made. It was created when Peter Pettigrew bumped into Ministry of Magic employee Bertha Jorkins in Albania while Voldemort was still weak.
Severus Snape: Doe
This indicates that Lily also had a doe patronus. Snape's patronus also leads Harry and Ron (Rupert Grint) to the sword of Gryffindor, though the boys don't know who it belongs to. The doe represents loving caretakers and determined protectors.