American blues singer Big Bill Broonzy (1903-1958) explained in a 1953 performance (and probably earlier):
 
“Some people call these folk songs. All the songs I’ve heard in my life was folk songs. I never heard horses sing one of ‘em yet.”
 
A 1956 newspaper attributed the remark to American jazz musician Louis Armstrong (1901-1971):
 
“Louis Armstrong, who’ll star on an NBC spectacular this Monday, was asked what he thought of folk music. ‘Folk music?’ Satchmo repeated. ‘Why, daddy, I don’t know of no other kind of music—I never heard a horse sing a song!’”
   
     
Wikipedia: Big Bill Broonzy
Big Bill Broonzy (June 26, 1893 or 1903 – August 14 or 15, 1958) was an American blues singer, songwriter and guitarist. His career began in the 1920s when he played country blues to mostly African-American audiences. Through the 1930s and 1940s he successfully navigated a transition in style to a more urban blues sound popular with working-class African-American audiences. In the 1950s a return to his traditional folk-blues roots made him one of the leading figures of the emerging American folk music revival and an international star. His long and varied career marks him as one of the key figures in the development of blues music in the 20th century.
 
Wikiquote: Louis Armstrong
Louis Daniel Armstrong (August 4, 1901 – July 6, 1971) was an American Jazz musician.
 
Sourced quotes
“All music is folk music. I ain’t never heard a horse sing a song.”
Simple: All music played and sang by people, I have never heard a horse sign a song.
 
Rambles
Big Bill Broonzy,
Amsterdam Live Concerts 1953
(Munich, 2006)
(...)
Amsterdam Live comes near to bringing Broonzy back to life. The sound quality is fabulous, so sharp and warm it sustains the illusion that Broonzy is talking, singing and playing to you personally. The songs, some repeated (because they’re taken from more than one concert), give a good sense of Broonzy’s broad stylistic range, even while he identified himself as a bluesman. There are lots of familiar old folk songs, plus early-blues standards such as Jim Jackson’s “Kansas City Blues,” Bessie Smith’s “Back-Water Blues” and Richard M. Jones’ “Trouble in Mind,” as well as some Broonzy originals (albeit not so many as one would like). All of this is interspersed with Broonzy’s good-natured reminiscences and wisecracks. He even recycles his often-quoted quip (usually and erroneously attributed to Louis Armstrong) that all songs are folk songs because horses don’t sing them.
 
12 September 1956, San Diego (CA) Union, “Point of View” by Donald Freeman, pg. A-16, col. 2:
Louis Armstrong, who’ll star on an NBC spectacular this Monday, was asked what he thought of folk music. “Folk music?” Satchmo repeated. “Why, daddy, I don’t know of no other kind of music—I never heard a horse sing a song!”
 
16 June 1957, The Observer (London, UK), “Skiffle Intelligentsia,” pg. 21, col. 4:
Ain’t Never Heard a Horse Sing
(...)
The going may be a bit rough, but as the great Louis Armstrong said recently, “They’re all folksongs. I ain’t never heard a horse sing.”
 
26 October 1959, Fort Lauderdale (FL) News, “Rover Should Be Wary Of Child With Mumps” by Hal Boyle, pg. 5-A, col. 2:
What kind of music do you like best?...says trumpeter Louis Armstrong: “All music’s gotta be ‘folk’ music: I ain’t never heard no horse sing no song!”
 
Fretboard Journal
Bob Shane, Big Bill Broonzy, Louis Armstrong and Horses and the Best Correction Ever
Michael John Simmons
April 2012
At the end of my Bob Shane story in issue 23 I quoted Louis Armstrong as saying, “All music is folk music: I ain’t never heard no horse sing a song.” Not long ago I got nice note from Chris Walz, the Department Chair of Bluegrss, Old-Time and Bluegrass at the Old Town School of Folk Music informing me that it was actually another music legend who said it. “The quote is actually from Big Bill Broonzy,” Walz wrote. “There are a couple of recordings where Big Bill uses that quote. One from a Studs Terkel radio interview, and one from a live concert recorded in England.” In his next email he identified Trouble in Mind as the CD that included the quote (it’s track 21, the spoken intro to “This Train”) and he kindly included a snippet of audio that included the quote, which you can listen to at the top of this post.
 
Twitter
Kent Gustavson, PhD‏
@drkent
“All of the songs I’ve heard in my life are folk songs - I never heard horses sing none of ‘em.” - Big Bill Broonzy #quotes
11:47 AM - 26 Mar 2013
 
Twitter
Newport Folk Fest‏
@Newportfolkfest
Replying to @stableford @danauerbach
“it’s all folk music, I ain’t never heard no horse sing” Big Bill Broonzy
6:52 AM - 24 Jul 2016
 
Twitter
Aaron Jaxon Band‏
@aaronjaxonband
“All music is folk music: I ain’t never heard a horse sing a song”-Louis Armstrong
1:03 PM - 17 Apr 2017