In reply to Brad, the article I linked to actually does mention that the metal horseshoes could have been a contributory factor to the incident. I think there is such a thing as plastic horseshoes, which would be non-conducting, but they are rarely used.
It has to be remembered that though the horseshoe is a good electrical conductor, it is fixed to a horse's hoof, which would be expected by contrast to be a fairly good insulating material. High voltage electric fencing is quite often used to keep horses in a field, and nobody would do that if horseshoes were so dangerous when it came to electricity.
When the Newbury electrocution incident was first reported in the media, some journalists tried to explain it as all being down to the horseshoes. Four horses were involved in the incident, and the two wearing steel shoes dropped dead whereas the two others wearing aluminium shoes survived, so the journalists were claiming they were saved by their aluminium horseshoes. But if the electrical conductivity of the horseshoes is the controlling factor, that's the wrong way round. Aluminium has a sixth of the resistivity of steel, so it is six times a better electrical conductor than steel.