Pixie
MOM Classification: XXX
(This article comes with a rather amusing note by Harry that reads “but XXXXXXX if you're Lockhart”)
So, Pixies are from Cornwall. They seem to be related to Imps and Fairies, except that they're more brightly-colored than an Imp (being bright blue) and they don't have wings (but can fly). They also stand out because they give birth to live young, unlike most other fairy creatures (besides Leprechauns).
Pixies are troublemakers and like to grab people by the ears and deposit them high off the ground, as they did to Neville in Lockhart's class, as well as pull other practical jokes. So they're fairly typical fair folk that way. They communicate with one another through a jabbering sound that only other Pixies can understand.
For the record, it isn't clear to me why they'd be from Cornwall specifically. Does anyone know?