Muggles' Guide to Harry Potter/Magic/Blibbering Humdinger

Overview
No description of the Blibbering Humdinger is ever given to us. From the fact that only Luna Lovegood seems to believe in its existence, we can safely assume that it springs from the fertile imagination of Xeno Lovegood, her father.

Extended Description
The Blibbering Humdinger is first mentioned in Harry Potter and the Order of the Phoenix when Luna Lovegood says that people used to think that they didn't exist. Hermione replies that they were right, because they didn't exist, causing Luna to slouch off, annoyed.

In Harry Potter and the Deathly Hallows, after the Battle of Hogwarts, Luna, seeing how tired Harry is, distracts the people around herself and Harry Great Hall by saying, "Ooh, look, a Blibbering Humdinger!" This allows Harry to don his Invisibility Cloak, and collect Ron and Hermione without anyone noticing.