A classic racetrack joke is:
 
“I bet on a wonderful horse. It took seven horses to beat him.”
 
The joke has been cited in print since at least 1942. New York-born comedian Joe E. Lewis (1902-1971) has long been associated with the joke. New York-based “one-liner” comedian Henny Youngman (1906-1998) has been associated with the joke since at least the 1970s, but there is no evidence that he originated it.
 
       
Wikipedia: Joe E. Lewis
Joe E. Lewis (January 12, 1902 – June 4, 1971), born Joseph Klewan in New York City, was an American comedian and singer.
 
Wikipedia: Henny Youngman
Henry “Henny” Youngman (original Yiddish surname Yungman; 16 March 1906 – 24 February 1998) was a British-born American comedian and violinist famous for his mastery of the “one-liner”. His best known one-liner was “Take my wife…please”.
 
18 June 1942, Atchison (KS) Daily Globe, “The Short Column,” pg. 9, col. 3:
Carl Mayhew:
“I once bet on a race horse. He came in sixth place. It took five horses to beat him. He was a wonderful horse.”
 
17 January 1958, Los ANgeles (CA) Times, “Ham on Ryon” by At Ryon, pg. 3, pg. 5, col. 1:
Clark (Halsey—ed.) was taking an awful beating until he cracked, “Sure, he didn’t win, but it took nine other horses to beat him!”
 
12 June 1958, Newsday (Long Island, NY), “Jester in Comeback Romp” by Lou DeFichy, pg. 19, col. 4:
Called From the Turf: Turk writer Toney Betts, who is as adept with a scungili fork as he is with a turf yarn, came up with the best yak of the meet. It seems there was a horseplayer who told his friend after one of the races: “I bet on the best horse in that race. It took 11 horses to beat him.”
 
9 October 1959, New York (NY) Herald Tribune, “Charles Ventura Reports,” sec. 2, pg. 3, col. 2:
Comedian Joe E. Lewis performed a gentlemanly act at a party given for Mrs. Stephen Stanford. One of the town’s well-known industrialists button-holed Mr. Lewis and said he had a brand new joke he could use in his current stint at the Copacabana. The joke: “I have a wonderful horse. It took twelve horses to beat him the other day.” Being an old Lewis fan, we remember when Lewis told that identical joke twenty years ago. However, he acted as though he had never heard it before, beamed and thanked the industrialist warmly.
 
1 June 1968, Chicago (IL) Tribune, “All in Sport” comic, sec. 2, pg. 5, col. 2:
“Jerome picked quite a horse. It took 10 other horses to beat him!”
 
15 August 1969, Chicago (IL) Tribune, “In the Wake of the News” by David Condon, sec. 3, pg. 1, col. 2:
“You guarantee Fast Hilarious is great, Joe (E. Lewis—ed.)?”
 
“I guarantee it, pal. Didn’t it take five horses to beat Fast Hilarious in the Round Table at Arlington back in July? Sure, I remember the day. It took five horses to beat him!”
 
16 November 1970, The Globe and Mail (Toronto, CA)< "Your morning smile," pg. 1, col. 6:

A bettor was thanking his friend for a race tip: “You said it was a great horse and it was—it took 11 other horses to beat him.”
 
11 June 1971, CHicago (IL) Tribune, “In the Wake of the News” by David Condon, sec. 3, pg. 3, col. 1:
Or, “I had a great horse at Arlington yesterday. It took 10 other horses to beat him.”
(From comedian Joe E. Lewis.—ed.)
 
2 July 1978, Boston (MA) Globe, “My Favorite Jokes” by Henny Youngman, Parade magazine, pg. 19, col. 2:
I bet on a horse the other day—it took seven horses to beat him.
 
Google Books
Vaudeville Humor:
The Collected Jokes, Routines, and SKits of Ed Lowry
Edited by Paul M. Levitt
Carbondale, IL: Southern Illinois University Press
2002
Pg. 127:
Bettor: That horse you gave me ran seventh. Absolutely last!
Tout: Shows you how good he was. Took six horses to beat him.
 
Twitter
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Henny Youngman: I played a great horse yesterday. It took seven horses to beat him. | #Quotes
7:23 AM - 13 Jan 2016