Here are some theories:
CHARLEY HORSE - "Back in 1946 the 'Journal of the American Medical Association' published an article entitled 'Treatment of the Charley Horse,' rather than 'Treatment of Injury to the Quadriceps Femoris.' This would indicate that 'charley horse' has been a part of formal English for at least 50 years. But did this term for a leg cramp arise from a lame horse named Charley that pulled a roller across the infield in the Chicago White Sox ballpark in the 1890s? That's the old story, and there was such a horse, but the expression may have been printed several years before his baseball days, in 1888, to describe a ballplayer's stiffness or lameness. Another derivation that seems likely but hasn't been proved traces 'charley horse' to the constables, or Charleys, of 17th century England. According to this theory, 'Charleys,' for 'local police,' survived in America through the 19th century and because aching legs were an occupational disease among Charleys, ballplayers suffering such maladies were compared to the coppers and said to be 'weary from riding Charley's horse.'" From "Encyclopedia of Word and Phrase Origins" by Robert Hendrickson (Facts on File, New York, 1997).