A joke about a flea circus is:
 
Q: What happened when the dog went to the flea circus?
A: It stole the show!
 
The joke has been cited in print since at least 1935, when it was credited to New York journalist Gene Fowler (1890-1960).
 
     
19 September 1935, Austin (TX) Statesman, “New York Day by Day” by O. O. McIntyre, pg. 4, col. 4:
And there’s Gene Fowler’s story of the Airdale that went to the flea circus and “stole the show.”
       
Google Books
The Railwayan
Volumes 23-25
1939
Pg. 36:
“Did you hear the big news?”
“Spill it.”
“My dog visited a flea circus and stole the show.”
 
7 July 1963, San Diego (CA) Union, “Fun Time—The Chuckle Box,” pg. A23, col. 3:
Mar; I took my dog to the flea circus.
Bill: What happened?
Mary: He stole the show.
   
Google Books
Open Highways:
A diagnostic and developmental reading program, Volume 5
By Helen Mansfield Robinson
Glenview, IL: Scott, Foresman
1966
Pg. 114:
Joe: I took my dog to a flea circus.
Bill: What happened?
Joe: He stole the show.
   
Google Books
The Compleat Flea
By Brendan Lehane
New York, NY: Viking Press
1969
Pg. 36:
‘And there was the one about a man sitting outside a flea circus with his dog that was scratching and another man asked the first man why he was scratching and he said, “He went in and stole the show”.’
   
Google Books
June 1974, Boys’ Life, “Think and Grin,” pg. 74, col. 4:
Then there was the dog who went to the flea circus and stole the show! — Sam Morris, Braintree, Mass.
 
Google Books
Doc Blakely’s Handbook of Wit and Pungent Humor
By James Blakely
Houston, TX: Rich Publishing Company
1980
Pg. 35:
Did you hear the story about the dog that walked into a flea circus and stole the show?
   
Google Books
Yo Mama Jama - Jokes For Kids 6
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
What happened when the dog went to the flea circus?
He stole the show.
 
Reddit—Jokes
Did you hear about the dog who went to the flea circus?
submitted April 16, 2015 by xplorer1701
He stole the show.
 
Twitter
Lucky Score‏
@LuckyScoreNet
Q: What
happened when the dog went to the flea
circus? - A: It stole the
show!
#Joke from #DogJokes in #Jokes,  #Humor and #Satire
2:20 AM - 8 Sep 2017