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.
A dead man with broken bones at a horse farm does not equal a "horse accident"...get real.
There are not 10 people who matter in Cook, Kane or lake county who buy that story