An acromantula is a fictional giant spider. Aragog and his family are the only acromantula mentioned in the Harry Potter books and movies. Their Ministry of Magic classification is XXXXX which corresponds to: known wizard killer/impossible to train or domesticate.
 Physical Appearance
Acromantula are described in Fantastic Beasts and Where to Find Them as "Monstrous eight-eyed monsters capable of human speech." They are said to have thick black hair which covers it's entire body and its leg span of up to fifteen feet.
 Appearances in the Harry Potter books and films
Acromantula first appear the the book and film adaptation of Harry Potter and the Chamber of Secrets when Harry Potter and Ron Weasly "follow the spiders" into The Forbidden Forest. They eventually find Aragog's lair and converse with him. The are subsequently chased from the forest by a hoard of hungry acromantula.
They also apear in both the book and film version of Harry Potter and the Half-Blood Prince.
See also Aragog
"Fantastic Beasts and Where to find them", J.K. Rowling, 2001