A giant spider in the Harry Potter novels. Harry encounters this loathsome being in Harry Potter and the Chamber of Secrets.
*If you haven't read further than Harry Potter and the Chamber of Secrets, spoilers ahead*
Raised by Rubeus Hagrid in his school days, Aragog got Hagrid into a bunch of trouble. Tom Riddle (aka Lord Voldemort in his later days), Head Boy of Hogwarts in Hagrid's third year, accused Aragog of being the mysterious creature hidden in Slytherin's mythical Chamber of Secrets.
Of course, this accusation was false. It didn't help much, since Hagrid had his wand broken in two, and spent a month or so at Azkaban, the Wizard prison inhabited by Dementors, until he was semi-cleared when Dumbledore spoke on his behalf and offered him the position of Gamekeeper at Hogwarts.
When Hagrid was accused, he sent Aragog away to live in the Forbidden Forest. Over the years her family grew and grew, and she remained faithful and friendly to Hagrid, for keeping her, bringing her food, etc. etc.
In Harry Potter and the Chamber of Secrets, when Hagrid is taken off to Azkaban again as a "precautionary measure" by Cornelius Fudge, because of the past, he says:
"If anyone wanted ter find out some stuff, all they'd have ter do would be ter follow the spiders. That'd lead 'em right! That's all I'm sayin'."
Harry and Weasley follow Hagrid's advice, and head out into the Forbidden Forest with Fang, Hagrid's dog. They follow spiders (lots of them), and Ron's arachnophobia shows. Eventually they reach Aragog's lair, where a giant arachnid awaits them. All of her children (oh! how many there were, it doesn't matter, either way Ron totally freaked out) were there with her, and after talking for a moment about Hagrid, basically tells her relatives to go have their dinner -- Ron and Harry.
Needless to say, our heroes escape by means of flying car, and escape Aragog. Perhaps he'll show up in a later book, but for now he looks like he's just a plot device to incriminate Hagrid, and tell Harry & co. that he's actually innocent.