Horse Pill Meaning

(idiomatic, by extension) A fact, proposal, claim, etc. that is difficult to accept or believe.

Example: 2002 June 26, "CNN.com - Dr. Sanjay Gupta: Vitamins and Alzheimer's," CNN (retrieved 25 Nov 2013):
  I'm lucky enough to remember to take my horse pill, much less drop it into a glass and wait 45 minutes.
2008 Dec. 31, Becky Jungbauer, "Vitamin Supplements: Fact Or Folklore?," Science 2.0 (retrieved 25 Nov 2013):
  I do know that I feel better when I remember to take my multivitamin, iron and vitamin D supplements, and the occasional fish oil horse pill.
1967 August 26, Harry Schwartz, "Answer To Soviet Shift: China," St. Petersburg Times, p. 8A (retrieved 25 Nov 2013):
  Even pro-Soviet Communists will find the Geneva accord a difficult horse pill to swallow.
1984 August 4, "Reagan will pledge to preserve tax cut, sources say," The News and Courier, p. 8A (retrieved 25 Nov 2013):
  Mac Carey, an aide to leading tax-cut crusaders in Congress, called the draft language in Reagan's radio speech "a horse pill. It's very difficult to swallow."
2002 June 20, Michael Cooper, "New York City Budget: The Deal," New York Times (retrieved 25 Nov 2013):
  Officials know that if this year's budget is a bitter pill to swallow, next year's will be a bitter horse pill.
2009 Nov. 30, Guy Junker, "Junker: Panthers need to get over it," triblive.com (retrieved 25 Nov 2013):
  They need to swallow that big horse pill that is the West Virginia loss, forget about it and re-focus this week.