The Invisble Garboil is a horrid monster which flattens Fraggles. It "appeared" in the Fraggle Rock episode "Gobo's Discovery." The Garboil, though itself invisible, appears as a foggy haze moving at rapid speeds through Fraggle Rock.
It is freed by Gobo accidentally after he stumbles into the rock pile keeping it pent up. The Invisible Garboil is later tricked back into its lair by Gobo.