There is less of a distance between a person's two feet than that of a horses front/back legs. Earth, the surface beneath our feet, typically has a high resistance. There is a higher chance "electricity" will choose to flow through a horse, as it has to either travel that distance through the horse, or through the ground. For a person, that distance is smaller, therefore less resistance, and less likely to use us a bridge between two spots on the ground.

Lol what? Google: Polonium-210 and lead-210 in the terrestrial environment: a historical review or Occurrence and Geochemistry of Lead-210 and Polonium-210 Radionuclides in Public-Drinking-Water Supplies from Principal Aquifers of the United States