An acromantula is a gigantic black spider (legspan may reach up to 15 feet) with a poisonous bite, the Acromantula is an anomaly in the beast / being classification system. While capable of human speech, the Acromantula is classified as a beast rather than a being due to its violent tendencies.
It is native to Borneo (FB), where it is believed to have been a wizard-bred species, serving as an example of why the Ban on Experimental Breeding was put into effect.
Note that all spiders, including Acromantulas, fear the basilisk above all other living things.
Aragog, Hagrid's acromantula friend, died in the spring of 1997 and was buried near Hagrid's hut (HBP22).
Acromantulas joined the Battle of Hogwarts, fighting for Voldemort (DH32).
"acro" from "akros" Greek, highest + "mantula" from "tarantula" Eng. a species of large, hairy black spider