horse around

beat a dead horse

To persist or continue far beyond any purpose, interest or reason.

change horses in midstream

To change one's plan or approach when an effort is already underway or at another inopportune time.

charley horse

A muscle cramp, usually a strong one, in the thigh, leg, arm, or core.

close the stable door after the horse has bolted

To attempt to prevent a problem that has already occurred; to design and implement a preventive mechanism that is too late (after the first failure instead of before).

dark horse

Someone who possesses talents or favorable characteristics that are not known or expected by others.

drive a coach and horses through

To spoil, break or render ineffective (a rule, plan or agreement).

enough to choke a horse

An excessive quantity.

flog a dead horse

To attempt to get more out of something that cannot give more; to attempt to arouse fresh interest in something that is either hopeless or already settled.