Luna Lovegood believes that Nargles infest mistletoe and stole her shoes. So, she keeps a butterbeer cork necklace.
But were Nargles a figment of her imagination?
Well, the Daily Prophet wrote about Nargles in somebody's wig. But, Rita Skeeter gave it a bad reputation for lying, which gave the Quibber a better reputation.
But the Quibbler writes about Crumple-Hormed Snorkack sightings. A Crumple-Horned Snorkack has not been seen throughout the whole series!
But let's go back to the beginning. Anybody could have stolen Luna's shoes (Draco Malfoy is on my list of possibilities.)
So, I say Nargles were a figment of Luna's imagination.