Aragog
Age: Died at the age of approximately 50 years.
Distinguishing Characteristics: Aragog is a giant spider and therefore quite easily distinguishable. The arachnid is blind with milky white eyes, and is greying with age.
Heritage: Acromantula
Family: A wife, Mosag, and several thousand children.
First Mention: Chapter thirteen of Chamber of Secrets (glimpsed in a memory). His first appearance is in chapter fifteen of Chamber of Secrets.
General: Aragog was hatched in a cupboard in Hogwarts, having been given as an egg to the half-giant (and then-student) Rubeus Hagrid. He grew up in this cupboard, living on scraps brought to him by Hagrid. Aragog's natural instinct was to feed on humans, but he did not do so while living in Hogwarts, out of respect for Hagrid.
However, when the creature from the Chamber of Secrets was released and attacked a student, Hagrid and Aragog were framed by Tom Riddle (the boy who would one day become Lord Voldemort and the person actually opening the Chamber). It was believed that Aragog was the creature that had attacked a student. He was forced to flee into the Forbidden Forest, and Hagrid was expelled.
Aragog remained in the Forest, feeding on the creatures there, and Hagrid was kept on as gamekeeper at Hogwarts. Thus, the two were able to remain in touch, and Hagrid found a wife for Aragog. Aragog and his new wife formed a large family of Acromantula, with Aragog as patriarch, relatively undisturbed by humans, except for a notable intrusion by Harry Potter and his friend Ron Weasley in early 1993. The Chamber of Secrets had again been opened, and Hagrid, who was back under suspicion and being taken to Azkaban, cryptically advised them to go to Aragog for help. Aragog related his story to Potter and Weasley, revealing Hagrid's innocence. Unfortunately, his children tried to eat them. They escaped with the help of Ron's father's flying car, which had gone wild and hidden in the Forest months before.
Aragog died of old age in 1997, following an extended period of illness during which Hagrid cared for him. However, upon his death the remaining Acromantula turned hostile toward even Hagrid, being no longer bound by Aragog's orders not to harm the half-giant. Although it is the tradition of Acromantula to eat their dead, Hagrid was able to escape the spiders with Aragog's body in order to give him a "proper burial." This send-off was also attended by Potter and by the Potions teacher, Horace Slughorn (who managed to steal some of the highly valuable venom from the body before Aragog was buried).
Aragog's extensive family continues to reside in the Forbidden Forest, now completely hostile to all wizards.