Acromantula

Acromantulas are giant, venomous spiders belonging to the world of Harry Potter. Dangerous and possessing a voracious appetite, they are described as one of the deadliest monsters to ever inhabit the world.

History
Endemic to the island of Borneo and southeast Asia, Acromantulas are thought to be a wizard-bred species designed to guard hoards of treasure and homes. They were first sighted on Borneo in 1794, and one was introduced to Scotland in 1943 after Rubeus Hagrid acquired Aragog's egg. Some time later a colony was established there after Hagrid gave the fellow Acromantula Mosag as a mate to him, presumably after having her smuggled into Britain like Aragog before her.

Acromantulas are covered under the 1965 Ban on Experimental breeding, designed to regulate the growth of wizard-bred creatures, due to their high toxicity, aggressiveness, the inability of wizards to tame it and its taste for human flesh.

The Chamber of Secrets
The book and film where these monsters make their debut.

The Prisoner of Azkaban
Ron Weasley fought a boggart  that assumed the appearance of an Acromantula during a Defense Against the Dark Arts class.

The Goblet of Fire
An Acromantula was placed inside the final Triwizard Tournament trial, the maze, where it was defeated by the combined efforts of Harry Potter and Cedric Diggory.

Biology and Behaviour
Acromantulas are described as enormous, hairy black spiders, with adults being as large as "carthorses" and their legs reaching a span of up to 15 feet. Each possesses eight large black eyes and a pair of chelicerae used to eat live prey and even their own dead kin. Additionally, they also have pincers, which are said to mainly be used for communicating as they create clicking sounds when agitated or excited. Highly aggressive due to their breeding as guards, they are known to eat humans and show a preference for "fresh meat." They are able to speak in human languages, but the only Acromantula known to have exhibited this trait was Aragog. Acromantulas also practice necrophagy, as they typically consumer family members after they die, though this seems restricted to their kin instead of their prey.

They also secrete an incredibly dangerous poison which retains its potency even some time after the Acromantula's death. A rare and sought after ingredient in potions and the like, it's valued at 100 Galleons a pint.

The females are larger than the male (as is typical for real spiders) and lay soft white eggs "as large as beachballs," up to 100 at a time. They hatch after six to eight weeks and are a Class A Non-Tradeable Material.

Originally from Borneo, they exhibit a preference for heavily forested areas and are able to adapt to hot, humid climes as well as cold weather (as they successfully established their presence in both Asia and Britain). Acromantulas build enormous white webs shaped like domes and live communally, which is unusual for spiders, since most carry out solitary lives and only come together to mate.

Their sworn enemy is the Basilisk, whose petrifying stare they fear since they possess eight eyes that they cannot close. Like all spiders, they flee from its presence, and their instinctual terror for it is so great that they forbid mentioning the monster in all conversation.

Trivia

 * Their names are composed by the words acros, Greek for "high" (referencing their tendency to build their webs high in trees),  and -mantula, from the word "tarantula."  However, in the film adaptation they resemble wolf spiders more than they do tarantulas.
 * Conversely, the Acromantula boggart that appeared in the third film looked like a black widow spider.
 * Their eight eyes symbolise the number eight in the wizard version of the runic alphabet.
 * According to the book Fantastic Beasts and Where to Find Them, the Ministry of Magic assigns a classification of five xs' to the species, meaning they are "known wizard killer(s)/impossible to train or domesticate," the most dangerous category out of all beasts.  In the book, the severely arachnophobic Ron Weasley penned in an additional nine xs'.
 * The Chamber of Secrets film featured a movie exclusive spell, "Arania Exumai (from latin words araneus exuo, "spider" and "I lay aside")", which was designed to fight or kill spiders. It was created by Tom Marvolo Riddle while still at school and later used by Harry Potter to defend himself and Ron against the pursuing Acromantula colony.