Green’s Dictionary of Slang

(US/P.R.) a male homosexual.

1966		H.L. Foster ‘Cant of the Disadvantaged, Socially Maladjusted, Secondary School Child’ in Urban Education Jan. 108: pato (duck in Spanish): homosexual.
1967		P. Thomas Down These Mean Streets (1970) 102: That cat ain’t a faggot, is he? [...] Sometimes these guys are patos and if you handle them right, you get the best breaks.
1972		B. Rodgers Queens’ Vernacular.
1981		H. Gould Fort Apache, The Bronx 105: Some of the kids in the crowd began quacking, and screaming, ‘Pato, Pato,’ the Spanish word for duck, which was Puerto Rican slang for fairy.
1993		A. Rodriguez Spidertown (1994) 1: Only patos wear that shit [i.e. aftershave] Is that wha’ thum blancos need t’ score some pussy?
1995		(con. 1985–90) P. Bourjois In Search of Respect 44: Who’s this little white boy who’s always calling here? Is he a pato [faggot] or something?