Explaining ‘Charlie Horse’ to an English language learner? Here's the story:

'Charlie' or 'Charley' used to be a common name applied to old horses, as in, ‘Old Charlie only has a few more trips to town left’ and, much like with older tall dogs, the poor old horses would commonly develop leg problems in the later life stages, making walking difficult. Websters dictionary confirms this saying ‘Charley was a common name for "old lame horses” kept for family use.’

This explanation from wordwizard.com, provides us with an interesting origin tale:

"In the 1890s, an old horse named Charley was used in the old Chicago White Sox ballpark to pull a roller across the infield. He was old and his muscles would get so stiff he could hardly walk. Players and spectators who caught a cramp thought of the old horse and started calling the condition a charley horse."