IO540 is right....and at voltages below 50V it becomes difficult to get enough current across a human body to kill, even with wet hands.
IIRC the standard for low-voltage power tools for hazardous environments is 48V, but I may be wrong.
115V is about the limit.... I've had the odd whack from 220V and 240V, but just been lucky.
Maybe a historian can tell us why America choose 100V (nominal, the 110V, then 115V, is just to account for the losses). Maybe that was OK for the insulation materials of the day, and by the time technology progressed, too many installations were in place to change?
Europe went for 200V (again 220V and 240V to account for the losses). Some low-voltage distribution systems went for 127V, which is the phase-to-neutral voltage for a three-phase network with 220V between phases.
As to cattle grids, yes, they're 5kV to 10kV but such a high internal resistance (that good old law of Ohm...) that they're not capable of delivering a mortal current, just a very nasty shock....
Sorry to hear about the horse.... I would have thought some sort of simple trip device in the supply could prevent that.