"The practical examination took place in the afternoon on the lawn on the edge of the Forbidden Forest, where students were required to correctly identify the Knarl hidden among a dozen hedgehogs (the trick was to offer them all milk in turn: Knarls, highly suspicious creatures whose quills had many magical properties, generally went berserk at what they saw as an attempt to poison them)" (OP31)
Very similar to a hedgehog, except for behavioural differences: a Knarl offended by an offering of food will destroy a garden in retaliation (FB). The quills of the Knarl have many magical properties (OP31).
The knarl takes offense easily and will think food left out my muggles is a trap and will wreak havoc on garden plants and ornaments (FB).
This question on the first W.O.M.B.A.T. test suggests that the Knarl is classified as a pest, since the most likely correct answer is Horklump:
23. Which of the Following is NOT listed as a pest by the Pest Advisory Board?
Part of the O.W.L. test in Care of Magical Creatures involved knowing how to distinguish between a Knarl and a hedgeghog (OP31).