Description
Nobody knows what a Boggart looks like when it is alone, because the moment anyone sees it, it becomes whatever that person fears most. Faced with several people at once, it can lose track of whose fear to take and come out as something muddled and half-formed.
Background
Boggarts like dark, enclosed places: wardrobes, the space beneath a bed, the cupboards under sinks, and crates left closed too long. They are not difficult to deal with, given nerve. The charm Riddikulus forces one into a ridiculous shape, and laughter finishes it off, which is why every third-year at Hogwarts is taught to face one.