Beating a dead horse...

[imported_goku]

Why beat a dead horse? Where did this expression come from? why not beat a dead ox? What is wrong with beating a dead horse anyways? I'm sure it helps relieve frustration, I see no problem in it, though I do feel sorry for the poor horse. Was the horse alive at one point but it was beaten to death? How does beating a horse used for transportation or carrying/moving (assuming thats what it's in reference to) has to do with politics or topics that are covered many times over?