Nagini didn't appear in Fantastic Beasts 3 - most likely because of Claudia Kim's pregnancy amidst the Covid-19 pandemic, but this also makes sense in-universe, as she couldn't have come close to
When Fantastic Beasts was announced, fans presumed familiar faces from Harry Potter would appear. However, Nagini was on nobody's list of characters they'd like to see in a prequel. Not because she wasn't well liked - it's that she didn't have enough of a character to have an opinion on.
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.
She's genuinely there for him every step of the way, and that means a lot to him. The Crimes of Grindelwald's climax sees Credence leave Nagini behind to join Grindelwald and learn his true identity. She then appears alongside Newt and his allies, implying she'll take their side in the upcoming war against Grindelwald.
There's been speculation he may have found her while in exile in Albania between 1981 and 1994, where he assumed a phantom-like form and possessed many animals (however, only snakes could bear him inhabiting their body). This could have something to do with his propensity to speak Parseltongue.
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.
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.
Nagini was a maledictus. That means that she carried an innate, hereditary blood curse giving her ability to transform herself into snake, which gradually became uncontrollable until Nagini eventually lost the ability to turn back into human completely.
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, 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). In the film, the witch is played by Claudia Kim.
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'.
Of course, Voldemort was a Parselmouth, and this was the root of his friendship with Nagini. Much like Grindelwald did with Credence, it's likely Voldemort gave Nagini a false sense of belonging after she had been trapped in a snake's body for possibly decades.
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.
After a careful investigation, the user suggested that it may have been Alison's makeup that made the difference. They added, “[It] really changes her face in my opinion; she's still beautiful if not more so.” More research proves that Alison was pregnant while filming, which could also explain things.
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.
Because of this, Nagini was born a female 'Maledictus' - one who carries the Maledictus curse. A 'Maledictus' suffers from a blood malediction- a type of curse that causes the sufferer to slowly transform in to an animal over the course of their life. The animal can vary according to the specific curse.
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.
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).
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."
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.
Nonetheless, despite her initially benevolent nature, Nagini's blood curse eventually turned her into a snake permanetly. 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.
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.
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.
"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."
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.