WordNet (r) 3.0 (2006):

horse opera
    n 1: a film about life in the western United States during the
         period of exploration and development [syn: Western,
         horse opera]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "horse opera":
   3-D, Cinemascope, Cinerama, Marchen, Technicolor, Western,
   Western story, Westerner, adventure story, allegory,
   animated cartoon, apologue, bedtime story, black-and-white film,
   cartoon, chiller, cinema, color film, creepie, detective story,
   documentary, documentary film, educational film, fable, fabliau,
   fairy tale, fantasy, feature, fiction, film, flick, flicker,
   folk story, folktale, gest, ghost story, horror picture, legend,
   love story, motion picture, motion-picture show, movie,
   moving picture, mystery, mystery story, myth, mythology, mythos,
   newsreel, nudie, nursery tale, parable, photodrama, photoplay,
   picture, picture show, pornographic film, preview, romance,
   science fiction, shocker, short, silent, silent film, skin flick,
   sneak preview, space fiction, space opera, spaghetti Western,
   suspense story, talkie, talking picture, thriller, trailer,
   underground film, whodunit, work of fiction