“A guy walks into a bar…”  is a typical form of what has been called the “bar joke.” A popular version involving a horse is:
 
“A horse walks into a bar and the barman says, ‘Why the long face?’”
 
A “horse walks into a bar…” anti-joke is:
 
“A horse walks into a bar. Several people get up and leave realizing the potential danger of the situation.”
 
“A horse walks into a bar…. Several people got up and left realizing the potential danger of the situation” was posted on Twitter on December 11, 2010.
 
       
Twitter
Spencer Gatlin
‏@TSGatty
A horse walks into a bar…. Several people got up and left realizing the potential danger of the situation.
12:55 PM - 11 Dec 2010 from Rohnert Park, CA
 
Twitter
Brent Kim
‏@BrentK86
A horse walks into a bar. Several people got up and left as they saw the potential danger of the situation.
5:02 PM - 28 Jan 2011
 
Twitter
Jake Duron
‏@TheJakeDuron
A horse walks into a bar. Several people get up and leave realizing the potential danger of the situation.
10:18 AM - 24 Mar 2011
     
ruk-si
Jokes
Last updated 2012-11-21 18:14
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A horse walks into a bar. Several people get up and leave because they realize the potential danger of the situation.
 
League of Legends
Anti jokes
Lunah
10-09-2013
A horse walks into a bar…
Several patrons notice the immediate possible danger in the situation, get up, and leave.
 
Tay’s Talk
30 OF THE BEST JOKES YOU’LL EVER READ
MARCH 17, 2015 TAYDEBEAR
(...)
5. A horse walks into a bar.
Several people get up and leave due to the potential danger of the situation.
 
Reddit—AntiJokes
A Horse Walks Into A Bar… (self.AntiJokes)
submitted October 18, 2016 by LotsOfJiggles
Several people stand up and leave due to the potential danger of the situation.