The Thunderbug is an insect/mimic-type enemy from Final Fantasy XII. It can be found in Crossover B in the Henne Mines. Thunderbugs, like other mimics, disguise themselves as treasure coffers and do not attack until inspected by the player. It is the only source of the loot item Charged Gizzard.
Being a battery mimic that, due to Mist exposure, has left the normal course of development and become something altogether different. As a result of the rapid changes induced by Mist, much of its form is twisted and misshapen. A residual static charge helps this creature to regulate its temperature, and generate the energy necessary to break down the materials from which it draws sustenance. Its body is an example of evolution gone horribly awry, and many natural philosophers have suggested that it is in a very much terminal state, seeing no possible further permutation of form through which this creature has not already been.
Curious indeed is this organ capable of capturing the very power of lightning. Made of metal, and yet flesh. Lines made of a metallic substance trace this gizzard like the strands of a spider's web, running thence throughout the creature's body like our veins, delivering energy where it is needed. These lines hold fast lightning, never relinquishing a drop while their host is whole. Natural philosophers call this organ the charged gizzard.