Recently the news has been full of horrible and terrible things — or, to be more precise, horribles and terribles. In last week's Senate hearings on Iraq, General David Petraeus and Ambassador Ryan Crocker outlined what might happen following a hasty withdrawal of U.S. forces, testimony that Barack Obama described as a "parade of horribles." Meanwhile, the actor Rob Lowe went public with an extortion attempt from a former nanny who he said was threatening to accuse him and his wife of "a vicious laundry list of false terribles." The entertainment blog Defamer sarcastically applauded Lowe's "keen ability to turn an adjective into a noun." Neither horrible or terrible are particularly new as nouns, but their latest appearances still merit a closer look.
The use of horrible as a noun meaning 'a horrible person or thing' is traced by the OED all the way back to 1400, with subsequent literary uses from DeFoe in 1726 ("Among all the horribles that we dress up Satan in") and Melville in 1851 ("Such a waggish leering as lurks in all you horribles!"). By the end of the 19th century, the noun horrible began showing up in the idiomatic phrase penny horrible, a cheaply published violent novel better known as a penny dreadful (or also sometimes penny awful). Parade of horribles also emerged in the late 19th century, originally referring to a procession of grotesques that was evidently popular in 4th of July celebrations around the U.S. The July 4, 1888 Boston Daily Globe ran an article previewing the day's festivities and prominently mentioned the "parades of horribles and sports for both young and old." The following year a notice ran in the Daily Nevada State Journal announcing a similar Independence Day procession in Reno.
As the Wikipedia page on the term describes, parade of horribles eventually got extended from its literal meaning to denote a rhetorical device in which the speaker hyperbolically catalogues the potential unpleasant outcomes of a course of action. In the context of the Iraq War, the expression popped up in War and Decision, the recently published memoir of Douglas J. Feith, former Undersecretary of Defense for Policy. In the book (and in an interview with "60 Minutes" on April 6), Feith described a 2002 memo written by Defense Secretary Donald Rumsfeld that contained "a comprehensive list of possible calamities in the event of military action against Iraq." (Many of these calamities have indeed come to pass.) "To relieve some of the tension inherent in the task," Feith wrote, "I began referring to the memo as the 'Parade of Horribles.'" It's possible that the coverage of Feith's book led Obama to crib the expression "parade of horribles" when characterizing the testimony of Petraeus and Crocker.
Rob Lowe's use of false terribles doesn't carry as much history behind it. True, the OED takes the noun terrible meaning 'a terrible thing or being' (usually in the plural) back to the early 17th century, but it hasn't had the same breadth of usage as the noun form of horrible. If Lowe's statement was written with input from his lawyer, you might expect false terribles to be a legal term of art. But I can't find any printed examples of that particular phrase before the Lowe story. Lawyers do sometimes talk about terribles, though, as in a Feb. 26, 2006 San Diego Union-Tribune article quoting Tim Alger, a lawyer representing the website Roommates.com: "The bottom line is that they [the San Diego Fair Housing Council] engaged in bullying tactics and sent us long letters threatening us with all sorts of terribles if we didn't buckle down and accept their version of what the law is." (Hmm, maybe Rob Lowe hired that guy.)
More generally, many adjectives ending in -able/-ible have spawned related noun forms: think of collectibles, convertibles, deductibles, disposables, intangibles, perishables, and unmentionables. Sometimes the noun overtakes the adjective: vegetable comes from an adjective describing something that is able to vegetate, i.e., grow like a plant. But forms like horribles and terribles will continue to raise eyebrows as long as people get rankled by enallage (or is it anthimeria?).
[Update: Aaron Dinkin writes in:
Beverly Farms, Massachusetts still holds what it describes as its "Horribles Parade" annually on the Fourth of July. I've never attended it (a different Fourth of July parade goes right past my parents' house, so I've never felt the need to go all the way to the Farms to see one), but I gather that it's no longer very much in the way of 'a procession of grotesques' so much as largely comic and satirical floats and so on. But the Horribles Parade dates to 1888, just in the period you refer to as the height of July 4th "Horribles" popularity.
Searching on the Boston Globe archive for "horribles parade" turns up a report on Independence Day festivities in Marblehead, Mass. from July 9, 1887. And a 1999 article said that Horribles Parades were still going on in the North Shore communities of Danvers, Winthrop, Gloucester, Salem, Beverly Farms, Manchester-by-the-Sea, and Marblehead. Going back to 1875, Boston had something called "The Antique and Horrible Parade" to commemorate the Battle of Bunker Hill.]