Negative Ned (Negative Nancy; Negative Nellie/Negative Nelly)

Entry in progress—B.P.
   
(Oxford English Dictionary)
negative, adj., adv.2, and int.
colloquial (originally U.S.). Preceding a forename beginning with N, forming a generic name for a person who is (habitually) negative, critical, or pessimistic. Esp. in negative Nelly and (in later use) negative Nancy.
Perhaps influenced by earlier uses, Nervous Nellie n. at nervous adj. and n. Compounds 2   and Nice Nelly n.
1934   Oakland (Calif.) Tribune 20 Aug. (Daily Mag. section) 1/8   ‘Another beauty of not going visiting,’ observed Negative Ned, ‘is that you don’t have to use those tom-thumb guest towels.’
1971   Berkshire Eagle (Pittsfield, Mass.) 11 Dec. 15/6   We’ve been swamped in a barrage of talk about what’s wrong with America, harangued by negative Nellies.
1999   Re: Did Anyone see This? in rec.arts.sf.starwars.collecting.misc (Usenet newsgroup) 13 May   Be prepared for one or two ‘negative-Nancies’ to try to pick apart your post.
 
Newspapers.com
20 August 1934, Oakland (CA) Tribune, “The Conning Tower” by F.P.A. (Franklin P. Adams), pg. 19, col. 8:
“Another beauty of not going visiting,” observed Negative Ned, “is that you don’t have to use those tom-thumb guest towels.”