The Neighs Have It
“Champing at the bit,” often mispronounced as “chomping” (champing means to grind or chew) refers to a racehorse impatiently chewing the metal bit in his mouth as he eagerly waits to start his race.
A trained eye can gauge the age of a horse by its teeth, so the saying “Don’t look a gift horse in the mouth” reminds those who receive gifts not to over-scrutinize them and seem ungrateful.
In politics and other fields, a “dark horse” candidate is comparatively unknown and obscure. The term began in 17th century horse racing and applied to little-known and hard-to-put-odds-on horses which were entered into races.
Who is this horse named Charley and why is your cramp named for him? At one time, “Charley” was a name for lamed racehorses. Many of these animals, however, got a second career dragging the dirt smooth at baseball games. According to one common origin story, players whose muscles cramped during play were compared to the limping equines, that is, the “Charley horses” and, by some accounts, it was one Charley in particular working for the Chicago White Sox.
Royalty, noblemen, and knights historically rode on tall horses, looking down on common people both figuratively and literally. Hence, telling someone to get of their high horse reminds them to stop acting superior.
Needless to say, beating (or flogging) a dead horse won’t get you more effort from the animal. However, there seem to be some surprisingly nautical connections for this idiom, too. British sailors were historically paid in advance, but often spent the money quickly, sometimes before their ship even left port. They got no more pay until the already-blown advance was worked off, or until “the dead horse was flogged.”
Horse traders were famously shrewd in their dealings, so the term “horse trading” for exchange and negotiation in politics came about in the early 19th century.