“East end of a horse going west” (a horse’s ass) is on many Internet lists of Southern expressions. “East end of a horse going westward” has been cited in print since at least 1901 and “east end of a horse which is headed west” since 1912. In September 1965, U.S. Senator Stephen M. Young (1889-1984) of Ohio wrote in a letter to a labor union leader who had threatened him, “Buster, you are the east end of a horse going west.”
 
The saying “uglier than the east end of a horse heading west” has been cited in print from about 2005.
 
 
5 April 1901, Wilkes-Barre (PA) Times, pg. 4, col. 2:
The “hare-brained” scribe who thinks the new courts house is a Quay scheme seems better acquainted—along with Farmer Bob—with the east end of a horse going westward than with honest criticism. He seems more taken up with apologies for wilful misrepresentations than in seeking after the truth.
 
Google Books
March 1912, The Typographical Journal, pg. 292, col. 2:
All of which would seem to account for the large number of sheepskin holders who spend their days contemplating the elevation of the east end of a horse which is headed west. Trade unionists can “point with pride” at the capacity of the graduates of their colleges.
 
Google News Archive
24 September 1965, Pittsburgh (PA) Press, pg. 24, col. 1:
East End Of A Horse
THE bullying tactics of labor leaders who are trying to push repeal of Section 14-B of the Taft-Hartley Act through the Senate this year have angered many people on Capitol Hill.
 
But it remained for a leading Democratic liberal, Sen. Stephen M. Young, of Ohio, to make the perfect retort.
 
Senator Young has supported many union causes through the years. Even so, he received what he described as an “insulting” letter on 14-B from Secretary William Cloud, of the International Typographical Union.
 
The letter, which went to many senators, quoted ITU President Elmer Brown as denouncing “dillydallying” and “doublecross” on 14-B repeal and threatening defeat to any senator who voted against it after accepting labor’s election help.
 
“Buster,” Senator Young wrote in reply, “you are the east end of a horse going west.
 
“In addition to this, you are a fool or an ignoramus in sending to senators of the United States the threatening assertions set forth in your letter.”
   
MMAFighting.net Forums
Thumpin
02-08-2007, 07:48 PM
(...)
“Uglier than the east end of a horse headed west.”
 
Google Books
Lizard Tales:
The Wit & Wisdom of Ron Shirley
By Ron Shirley
East Stroudsburg, PA: Eme Press
2010
Pg. 127:
Uglier than the east end of a horse heading west.
 
Breakfast to Bed
His Mouth Runs Like A Pigeon’s Butt.
Posted on February 18, 2012 by breakfasttobed
(...) 
“He’s uglier than the east end of a horse headed west.”
 
Why oh why don’t we have sayings like this in NYC?