An Imp is a fairy that was kept captive for too long (one night) without being outside. It has a tarantula-like appearance and has an evil nature. The Imp can be the size of a fairy or can be taller than a human. An imp can be changed back into a fairy if a fairy kisses it on the lips. Then it gains the vain, conceited nature and prettiness of a normal fairy. It will remember being an imp, but will think that their behavior was silly. You can read about these more in Fantastical Beasts and Where to Find Them by Newt Scamander or in the Fablehaven series by Brandon Mull.