OP - If the horses are getting shocked, there is a defect in the heater, or a crack in the wire. You aren't putting the connection of the extension cord and the heater in the water, right? That would cause a shock too.
Do you have electric fence and have the cord for the heater touching one of the fence wires? Quick story: I was filling one of the tanks and had the hose on one wire of the fence. Up came one of the horses to get a drink. As he touched the water, I heard the fence zap. It traveled along the hose to the water and thru the horse. He was hesitant to get another drink. I couldn't help laughing. Now back to your issue.
Last year we used a floating heater. It did ok but it only worked for that year. This year it didn't heat as well and there was some ice in their tank. The other problems we had were the cord wouldn't stay straight so the heater was always on the far side from the horses and the boys would play with the cord. They never pulled the heater out but could have.
This year we went to using one that goes through the drain plug. They work great and keep the whole tank ice free. Only the heating element is in the tank. No wires exposed to the horses or water.
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