To do this, you need to create however many horse spawns you want/need. Then, you create a separate brick with the events "on bot touch>bot>change datablock>normal zombie". Then, get on a horse, touch the brick, and repeat.
Eventually, if you stack them high enough, the zombies will bend through the floor of the map, and come back up, like a loop. I'm not sure what happened at the end, though.