Parade of horribles

A parade of horribles is both a literal parade and a rhetorical device.
As a literal parade.
The phrase "parade of horribles" originally referred to a literal parade of people wearing comic and grotesque costumes, rather like the Philadelphia Mummers Parade. It was a traditional feature of Fourth of July parades in parts of the United States in the 19th century, and "Horribles Parades" continue to be part of the Independence Day celebration in several New England communities. A 1926 newspaper article about July the Fourth celebrations in the White Mountains of New Hampshire notes:
Old-time celebrations are to be held tomorrow at Littleton, Lancaster, Colebrook, and Conway, with all the usual features of street parades of horribles and grotesques, brass bands, decorated automobiles and vehicles, exhibitions by fire departments, basket picnics in convenient groves...
Founded in 1926, the Ancient and Horribles Parade in Chepachet, Rhode Island continues this tradition.
Other rural New England towns, such as Hopkinton, Massachusetts, and Mendon, Massachusetts, still hold annual Horribles Parades.
As a rhetorical device.
A parade of horribles is also a rhetorical device whereby the speaker argues against taking a certain course of action by listing a number of extremely undesirable events which will ostensibly result from the action. Its power lies in the emotional impact of the unpleasant predictions; however, a parade of horribles can potentially be a fallacy if one or more of the following is true:
A parade of horribles is a type of hyperbole, because it exaggerates the negative results of the action, arguing that "If we do this, ultimately all these horrible things will happen".