Nagini

Nagini is the pet Snake of Voldemort in the 'Harry Potter' series. She first appeared on screen in the film adaption of Harry Potter and the Goblet of Fire. She appears at the beginning of the movie and tells Voldemort, Barty Crouch Jr. and Wormtail that Frank Bryce is outside of the door. This lead to Frank's death.

Many didn't know until J.K. Rowling announced it, but in Harry Potter and the Sorcerer's Stone, Harry release a snake in the zoo. This snake was the same snake that later became Nagini.

In the seventh and final book, Voldemort uses Nagini to possess Bathilda Bagshot, who was killed earlier. He knew that Harry Potter was coming to Goderic's Hallow. Later, Harry indeed arrives at Goderic's Hallow with Hermione. Nagini, In Bathilda's body, brings them to Bathilda's house and takes Harry upstairs into a dark room. There she contacts Voldemort and Voldemort tells her to keep Harry with her. Nagini left Bathilda's body and attacked Harry. Harry yelled for help and Hermione came to the rescue. Later in the same book, Nagini is killed by Neville Longbottom during the battle of Hogwarts. Neville did this since Harry ordered him to kill Nagini because she was the last horcrux remaining.

Nagini is also responsible for the death of Severus Snape, after Voldemort ordered her to kill him.

Trivia

 * Bathilda Bagshot is portrayed by Hazel Douglas in Harry Potter and the Deathly Hallows: part 1. But since Hazel only portrays Bathilda in the scenes where she is possessed by Nagini, it could be said that she portrays Nagini as well.
 * In the movie version of Harry Potter and the Deathly Hallows: part 1, Harry hits Nagini on the head with a stone. This never happened in the book.
 * There is said that, in Harry Potter and the Deathly Hallows: part 2, Nagini will chase Ron and Hermione around the castle, this also never happened in the book.
 * Nagini's species is never revealed in the books- she is stated to be venomous, but much larger than any venomous snake in real life (with the possible exception of the King Cobra, but the books never mention any cobra-like features, such as a hood). It is most likely that she is a magical species unknown to muggles, perhaps even created by hybridizing several species of snake. In the movies she is portrayed by a CG snake, but her design is heavily inspired on pythons- Burmese python in Harry Potter and the Goblet of Fire, and Reticulated python in Harry Potter and the Deathly Hallows (both parts).
 * In Harry Potter and the Deathly Hallows: part 2, both Ron and Hermione try to kill stab Nagini with a fang from the basilisk. They both fail.