Luna Lovegood is always blabbing about Narguls stealing her things and causing mischeif. Does anyone know what narguls are?

The Nargle is yet another creature that Luna Lovegood and Xenophilius Lovegood believe exists though not many others do. The Nargle is known to infest mistletoe and to be a mischievous thief. Luna Lovegood claims that her Butterbeer cork collar keeps away the Nargles.

from link

Nargles are invisible creatures that go inside your head and make your brain go fuzzy, as Luna frases in book 5.