Aragog -acromantulas-

Aragog was an Acromantula - an enormous talking spider that made a unique clicking noise as he moved in search of prey. Hagrid raised him from an egg , keeping him inside a cupboard in Hogwarts. The spider's discovery was partly responsible for Hagrid's expulsion. Hagrid was caught talking to the spider in the dungeons by Tom Riddle, Riddle then maked up that the Aragog was the "Monster of Slytherin," and that Hagrid, was the one who opened the Chamber of Secrets and released it. Actually, the "Monster" was a basilisc which Riddle, the real Heir of Slytherin, had released.

After Hagrid's expulsion, Aragog lived in the Forbidden Forest. Hagrid even found him a mate, Mosag, with whom Aragog had many offspring and became the patriarch of an entire colony of Acromantulas. He remained grateful to Hagrid and kept his carnivorous children from attacking him when he came to visit, but this did not extend to anyone else (as Harry, Ron, and Fang found out in Chamber of Secrets). Hagrid had told them to simply "follow the spiders." Doing as he wished, they found Aragog and his children, who tried to eat them. They were only saved at the last minute by Ron's flying car that had been lost in the forest a few months prior. Aragog remained in the forest for the rest of his life, eventually growing weak, going blind, and dying in the Half-Blood Prince. Hagrid retrieved Aragog's body from the forest, fearing that his children would devour his body. During his funeral, Professor Slughorn bottles his precious venom and takes it for himself.

Thanks for the info, Rodro!
I didn't know what the name of his mate was.