A hidden creature is a creature unknown to the general public, either by disguising itself well enough to hide in plain sight, blinking it's body out of Usual Existence, or by attacking and sometimes killing anyone who tries to reveal it to the general public.
These in turn are given three classifications.
Clandestine, for those who choose to disguise themselves.
NOTE: Clandestine creatures can only disguise themselves to the main human eye. Peripheral vision breaks through the disguise, and shows the true form of the creature. Some creatures are beginning to work around this flaw, the more intelligent ones.
Ethereal, for those who choose to leave the Usual Existence.
Update: With some thought, I've decided to add a new classification. Hostile. This splits malignant into two, essentially.
Hostile, for those who choose to attack and kill humans in defense or for the sake of remaining hidden.
Malignant, for those who choose to loathe and envy humans rather than hiding in peace.
If a creature falls under one of these classifications, and is unknown to the general public. it is a hidden creature.
This system, as of now, is still flawed. Because undiscovered species with the ability to disguise themselves, such as a new species of stick bug, would technically count as hidden creatures.