Kappa - Japanese creatures that live in rivers, ponds and lakes, but they will wade ashore in eager search of their prey if the prey doesn't wade out to them. Either way, they drown their victims and mutilate them, drinking their blood and eating livers, of which they are especially fond.

The adult kappa is roughly the size of a ten-year old child. It has yellow-green, scaly skin, a monkey face and the back of a tortoise, as well as webbed feet. There is a saucerlike depression on the top of its head, which must always contain water or the creature loses its supernatural abilities while it walks on land. A kappa may be defeated if one bows to it repeatedly, causing it to bow back and thus having the liquid spill out of the depression. This forces the kappa to go back to its watery home, without harming anyone.

Kelpie - a Celtic water demon, appearing as a horse with a mane of green rushes. This creature tempts people to ride it, then drags them into deep water to drown them. If one is bridled, however, it will be subdued and the power of its supernatural strength can be harnessed; the kelpie can do the work of several horses.

Killing Curse - the third of the three Unforgivable Curses, also known as Avada Kedavra

Knarl - inhabits northern Europe and America. Commonly mistaken for a hedgehog by muggles, but there is one major difference: while the hedgehog will accept food left out in the garden for it, the knarl assumes the food is a lure into entrapment, and vandalizes the homeowner's plants and/or garden ornaments.

Kneazle - originally from Britain, a small catlike animal that is now a worldwide export. Its fur is spotted or flecked, its ears large, and its tail resembles a lion's. They can breed with cats. They are characterized by intelligence, independence, occasional aggression, and are capable of being wonderful pets if they like a wizard or witch. Kneazles can detect suspicious people very well and react badly to them.

Knotgrass (Plant) - a Polyjuice potion ingredient.