what are nargle's?

Ask Luna she knows :)
Im not sure but I guess they're some kind of magical creatures.However I dont know if they exist या they're part of her imagination

They're invisible creatures, the float in your ears and make your brain go fuzzy!
Luna Lovegood-Harry Potter and the Half-Blood Prince.

Nargles are magical creatures. Nargle's is the possesive tense of Nargle.
-Grammar Dementor

The Nargle is a creature known to infest mistletoe and to be a mischievous thief
-harrypotterwiki-

Haven't आप heard? Nargles are very sheepish creachers that are relatives to the leprechauns they too are small in size and प्यार mischief and are found in mistletoe where they can cause the most damage. No one has ever seen they for they are invisible but they like leprechauns tend to wait for the opportune moment to do destructive things.