Harry Potter Animal
Owner Lord Voldemort
Special powers Possible Horcrux
First appearance Harry Potter and the Goblet of Fire
Nagini is a fictional animal from the Harry Potter series. She is a giant snake and a loyal pet of Lord Voldemort, who as a Parselmouth, is quite capable of communicating with her.
Nagini seems to be the "evil" equivalent to Albus Dumbledore's phoenix Fawkes. Even more so, however, she seems to be Voldemort's spiritual familiar, sharing an intimate bond with her master. Nagini is the female name for cobra in Hindi and most other Indian languages. In Hindi, the word is pronounced /nαːginiː/.
Spoiler warning: Plot and/or ending details follow.
During the fourth year Harry spends at Hogwarts, Voldemort's temporary body was sustained by Nagini's "milk" (presumably her venom), harvested by Peter Pettigrew (aka Wormtail). After Voldemort returned to his former glory, the snake remained in his service, being both well-treated and well-used.
Readers are first introduced to Nagini in the first chapter of Harry Potter and the Goblet of Fire. In that chapter, Voldemort, Pettigrew, and Nagini are living in an abandoned house discussing their plot to kill Harry Potter. Nagini is the first to notice Frank Bryce, the house's gardener, and uses the language parseltongue (the language of snakes) to communicate with Voldemort and alert him to Bryce's presence. Bryce is then killed.
In the fifth book, Harry sees Nagini attacking Arthur Weasley in one of his dreams. He saw this from Nagini's point of view, as Voldemort was possessing her at the time.
In the sixth book, Dumbledore speculates that Nagini may be one of the six Horcruxes that Voldemort left behind to achieve immortality.
It is unknown what true species of snake Nagini is, though she is believed to be venomous, due to the fact that Voldemort refers to her donating her venom to the potion that brings him back to life in the fourth book. In Order of the Phoenix it is also revealed that her venom makes it very difficult for wounds to heal, and melts stitches. The fact that she has a hood suggests that she is a cobra (Naja naja or Naja hannah). Given her size and the unusual properties of her venom it is highly likely she is a magically altered or wizard-bred variety of cobra.