The “horse opera” was an early name for the western (in radio and on the movie screen), dating from at least 1923.
 
The term “horse opera” had long been in use before that, meaning an entertainment featuring horses. This “horse opera” use dates from at least the 1840s. Adam Forepaugh’s circus (a rival of P. T. Barnum) contained Wild West acts and used the term “horse opera” in the 1870s.
 
The term “soap opera” (for dramatic serials) originated later—from 1939.
 
 
The Circus in America
Circus: Adam Forepaugh Circus, 1867-1894
The Adam Forepaugh Circus, founded by Adam Forepaugh, was a major competitor of P.T. Barnum and Ringling Bros. Recognized for innovation in a highly competitive industry, the Forepaugh Circus initiated a dual roundtop system, dividing the menagerie from the circus performance. Thousands of audience members convened under a half-mile round tent to see spectacles like “The Light of Asia” (a “white” elephant), “Battles of the War for Freedom,” and “ ‘Jack’ the Only Boxing Kangaroo.” While independently successful for twenty-seven years, audiences grew wary of his shows, where grifting, short-changing and pickpockets ran rife. Forepaugh’s unscrupulous business practices created a market niche for “Sunday School Shows” like Ringling Bros. and P.T. Barnum, shows that would eventually outshine and outlast the Forepaugh Circus.
Alternative Names:
Forepaugh’s Circus
Great Forepaugh Show
Adam Forepaugh Circus, Adam Forepaugh Sr., Proprietor
Forepaugh & The Wild West
   
(Oxford English Dictionary)
horse opera colloq. (orig. U.S.), a ‘Western’ film or television series
1927 Motion Picture Classic 2 July 26/1 *Horse Opera..is an opus of the West where men are cowboys.
(...)
soap opera
colloq. (orig. U.S.).
A radio or television serial dealing esp. with domestic situations and freq. characterized by melodrama and sentimentality; this type of serial considered as a genre.
[1938 Christian Cent. 24 Aug. 1011/1 These fifteen-minute tragedies..I call the ‘soap tragedies’..because it is by the grace of soap I am allowed to shed tears for these characters who suffer so much from life.] 1939 Newsweek 13 Nov. 44/2 Transcontinental Network bubbled up out of the ‘soap operas’.
           
(Historical Dictionary of American Slang)
horse opera n.
1. an entertainment featuring trained horses; (hence, disparagingly) a circus or carnival.
[1857 in Dictionary of Americanisms: The denizens of the Bowery, who prefer the equine opera, will do well to make the most of present opportunities.}
1864 in Dictionary of Americanisms: Those fond of “horse opera”—and who is not?—will have an opportunity to gratify themselves—by visiting the Pavilion.
1867 in Dictionary of Americanisms: Of course all our people, old and young, will visit the “horse opera.”
1931 Amer. Merc. (Nov.) 352: Horse opery, n. Jocular for any circus.
   
2. a cowboy film, esp. if undistinguished; (hence) a radio or TV western.
1927 in OEDS: Horse Opera…is an opus of the West where men are cowboys.
1928 N.Y. Times VIII (Mar. 11) 6: Horse Opera—A western cowboy picture.
   
8 September 1847, Weekly Wisconsin (Milwaukee, WI), pg. 3, col. 3:
”...but you have geeve me von dem tickets to de Opera la Cheval—vot you call de horse opera—de dem Sarecuss.”
   
18 April 1860, New York Times, pg. 4, col. 5:
NIBLO’S GARDEN.—The latest novelty at this establishment is a veritable “horse opera”—produced on Monday night with unbounded success. It is none other than AUBER’s “Bronze Horse,” adapted and mounted for equestrian purposes, but retaining much of the music and nearly all of the striking situations of the original version.
 
Making of America
July 1863, Atlantic Monthly, pg. 90:
She was quite ashamed that he detected her once in going to the Horse-Opera, he must think her taste so low.
     
13 November 1868, Galveston (TX) Daily News, pg. 1, col. 3:
The circus is coming—Ames’ Circus, of New Orleans. It will be here next week. Garry Oldis, advertising agent for the concern, is already here. A small menagerie is connected with the horse opera, in which the “king of beasts,” the lion, and his master, the elephants, figures conspicuously.
 
Making of America
9 July 1870, Punchinello, pg. 229:
The OATES troupe now performing at the Olympic Theatre must not be confounded with the Horse Opera. 
 
23 August 1877, Nevada State Journal (Reno, NV), pg. 3, col. 3:
Not a few noble red men and a sprinkling of Chinamen completed the throng that lined our streets yesterday—and all because 4 paws big “horse opera” was here. When a circus ceases to draw, then comes the millennium, sure.
 
9 June 1910, Van Wert (OH) Daily Bulletin, pg. 2, col. 2:
The reigning favorites among Prima Donnas of horse opera. The leading proponents of equilibrism upon the bare backs of swift horses.
   
10 July 1910, Washington Post, magazine section, pg. 3, col. 1:
“‘Truth is stranger than fiction’” is an old and rather trite saying but it goes just the same,” said an old-time advance agent who has piloted everything from horse opera to grand opera through the country for a great many years, to a group of friends in a Broadway cafe the other evening.
 
17 October 1917, Lincoln (NE) Daily Star, pg. 7, col. 4:
Vola Vale, the only girl playing in William Hart’s second Autocrat picture,has been elected by Mr. Hart and his thirty cowboys as an honorary member of the “Horse Opera Troupe” as Mr. Hart’s company is called.
   
16 April 1918, Fort Wayne (IN) Journal-Gazette, pg.22:
“The Hammerstein of the Horse Opera.”
 
6 February 1923, Los Angeles Times, pg. II11:
Colin Campbell is about to start work on “The Grail,” a new Fox feature, which promises to rival “The Spoilers,” which Campbell made some sever years ago, and which is again to be done by Hampton.
 
“‘The Grail’ is a western story,” explained Mr. Campbell, the other day, “but it isn’t a ‘horse opera.’ It is a very big, human story.”