Acromantulas are a fictional sapient species featured in the Harry Potter series.
Acromantulas are a sapient breed of spiders who can grow up to be "as large as carthorses". They are strict carnivores, but not naturally ravenous, though most Acromantulas will turn man-eating if left alone in the wild. Acromantulas show their emotions through the position of their pincers, which they will click together when excited. On top of their great strength, Acromantulas possess a deadly (and highly valuable) venom, and are resistant to most spells except for the dedicated counter-curse "Arania Exumai".
Acromantulas were created by South-Asian wizards from regular spiders as artifical guardians for their treasures. Thanks to their humanlike intelligence, the first Acromantulas outwitted their creators and returned to the wild, living and hunting in the thick rainforests that hid them from manking. They were rediscovered in 1794, and the explorers also made the painful discovery that Acromantulas had acquired a taste for human flesh.
A firm ban on the trade of Acromantula eggs was instituted, but this did not stop Rubeus Hagrid, a young British wizard with an interest in magical creatures, from acquiring one, whom he called Aragog. Hagrid did his best to educate Aragog, teaching him proper English and a moral code, until fellow student Tom Riddle (later to be known as Lord Voldemort) framed Aragog for a murder actually committed by Riddle's Basilisk. To save him from certain death, Hagrid sent Aragog to hide in the Forbidden Forest near Hogwarts Castle. There, Aragog found a mate, Mosag, whom Hagrid naively smuggled into the Forest. Together, they had many children, founding a real colony. However, Aragog was blinded in a terrible accident and became unable to properly raise his own spawn, who became savage and refused to abide by their father's command of never eating human flesh.
During the Battle of Hogwarts, Lord Voldemort's followers took over the Acromantulas' colony as a temporary base. Disgruntled, the spiders, under the lead of their queen Mosag, then joined in on the battle in retaliation, though most could not tell wizards apart and attacked both Death Eaters and their own human "allies".