Cornish Pixies are small, electric-blue creatures with a love of troublemaking. In the first Defence Against the Dark Arts class of Harry Potter and the Chamber of Secrets, Professor Lockhart lets loose a cage of the creatures and leaves the class to deal with them.
From the Story
"Electric blue with pointed faces and shrill voices, Cornish Pixies may only be eight inches high but they can cause a great deal of damage. They are mischievous creatures that delight in cruel jokes and causing trouble."
On Pottermore, there is a minigame where the player has to catch as many pixies as possible (with a maximum of ten) in only 50 seconds.