Parade of horribles
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...
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:
- The action doesn't actually change the likelihood of the "horribles" occurring. The "horribles" could be unlikely to occur even if the action is taken, or they could be likely to happen anyway even if the action is avoided. This is an appeal to probability, and can be viewed as a non sequitur insofar as the action has no causal relation to the "horribles".
- The argument relies solely on the emotional impact of the "horribles" (an appeal to emotion).
- The "horribles" are not actually bad.
- The "horribles" have a low probability of occurring when compared to the high probability of good occurring.
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".
- "Horribles Parades a weird tradition." The Boston Globe, July 4, 1999
- "Mountain Season Early. Camp and Stream Bring Many Visitors to New Hampshire Despite Coolness". The New York Times, July 4, 1926, p. X8
- Brown, Seth. Rhode Island Curiosities: Quirky Characters, Roadside Oddities & Other Offbeat Stuff (Globe Pequot, 2007)