Which Creature May Not Bow Its Head?

HARRY LEARNS ABOUT KAPPAS, “CREEPY,
water-dwellers that looked like scaly monkeys,” in Rowling’s
Harry Potter and the Prisoner of Azkaban.
They are mentioned again in Beasts.
J. K. Rowling did not invent kappas. They are,
as she says in Beasts, water demons of
Japanese legend. (In Harry Potter and the
Prisoner of Azkaban, Professor Snape makes an error when he
informs Harry’s class that “the kappa is more commonly found in
Mongolia.”) Like the grindylows who live near Hogwarts, kappas live
in lakes and rivers, and capture people in the water. They are also
known as Kawako, which means “Child of the River.”
The description in Beasts, odd as it sounds, is true to legend. Kappas
can be vicious and enjoy the taste of blood.

See also: Beasts Grindylows
Merpeople
However, a human may escape from a kappa by
exploiting the creature’s greatest weakness. Its vitality is drawn
from a saucer-like depression on its head, which must remain filled
with water. If one offers a polite and ceremonious bow, the kappa
will be obligated to return it. The water will spill and the kappa
will be defeated. As well, for some long-forgotten reason, kappas
love cucumbers almost as they love human blood. A gift of a
cucumber may win the friendship of a kappa, who might then reveal
secrets about medicine.