Wikia

A classification is given by the Department for the Regulation and Control of Magical Creatures to all known Beasts, Beings and Spirits. These offer an at-a-glance guide to the perceived dangerousness of a creature. There are five categories.

The Five CategoriesEdit

XXXXX: Known wizard killer / impossible to train or domesticate.
XXXX: Dangerous / requires specialist knowledge / skilled wizard may handle.
XXX: Competent wizards should cope.
XX: Harmless / may be domesticated.
X: Boring.

In Fantastic Beasts and Where to Find Them, Ron Weasley adds nine "X" to Acromantula's Classification. This is a reference to Ron's fear of spiders. This may also be contributed to by Aragog's offspring almost killing them, only being saved by the chance appearance of his father's car.
Despite the Basilisk being a XXXXX-ranked creature, meaning they are impossible to be domesticated, Parselmouths are able to control them. This is likely due instead to the incredibly dangerous nature of the creatures.