Aragog

Aragog was an Acromantula, an enormous, sentient spider capable of speech, who made a unique clicking noise as he moved in search of prey. Hagrid raised Aragog from an egg as a Hogwarts student, keeping him inside a cupboard. In his third year at Hogwarts, Hagrid was caught talking to Aragog in the dungeons by Tom Riddle, who alleged that Aragog was the "Monster of Slytherin," and that Hagrid had opened the Chamber of Secrets. In fact, it was Riddle who had opened the Chamber, and the monster was actually a basilisk.

After Hagrid was expelled, Aragog lived in the Forbidden Forest. Hagrid found him a mate, Mosag, with whom Aragog bore an entire colony of giant spiders. He remained grateful to Hagrid for his entire life, and kept his carnivorous children from attacking him when he came to visit. However, he did not extend this courtesy to other creatures and people; he allowed his children to attack Harry, Ron, and Fang when they encountered him in Harry Potter and the Chamber of Secrets. Hagrid led them to Aragog by giving them the cryptic advice, "follow the spiders." It was Aragog who told Harry and Ron that he was not the Monster of Slytherin, but another creature, that unlike Aragog, was born in the castle, and which spiders so fear above all others that they never speak of it or give its name, despite Hagrid's numerous inquiries. Though Aragog allowed his children to attack Harry, Ron, and Fang, they were saved at the last minute by Ron's father Arthur Weasley’s flying car, which had been lost in the forest months before. Aragog remained in the Forbidden Forest for the rest of his life, but eventually died of old age in Harry Potter and the Half-Blood Prince. Hagrid retrieved Aragog’s body from the forest so that he could give him a proper burial, fearing that his children would devour his body. From that point on, the spider colony was the only part of the Forbidden Forest that Hagrid could not enter safely.

Aragog’s children returned in Harry Potter and the Deathly Hallows during the Battle at Hogwarts; having been driven from the forest, they began attacking Death Eaters and Hogwarts' inhabitants indiscriminately. Hagrid’s love of the spiders endangered him and others because he tried to protect them; the spiders thanked Hagrid by capturing him and taking him to Voldemort.

Aragog was voiced by Julian Glover in the film adaptation of the Chamber of Secrets.