Oh, I thought you had switches Improved Unarmed Strike for some archery feat or another, but now I remember you did not... Okay, well first you need to move your horse to a free space and the only one I can really see right now is between d7 end e8 and will require an overrun on the zombie in f7, but this would allow you to kick the zombie in g7...
Edit: the prone zombie in d6 would get an attack of opportunity (most likely on the horse because he's prone) because you'd be moving through his threatened area, but that should not be much of a problem since he would be at -4 on this attack because he is prone...