No offense but it would be virtually impossible for a hit man to attack a person in a way that was consistent enough with a fall off a horse so as to trick a medical examiner.

These guys aren't expert assassins or anything.

Plus the mob generally puts a few in someone's head just to make sure they don't make it and cause a whole bunch of problems.

The idea that a hitman scared someone off a horse, broke his back or neck or whatever in a way that could fool an ME, then left the guy there, still living, is preposterous.