I have had problems with horses and bees. Although the horses were
seperated from the bees by electric fence the horses were in a half acre
area.  I think the horses when stung need to be able to move well away from
the hives so that additional bees don't sting due to the alarm pheremone. In
addition a barrier the bees must fly up and over so the horses are not in the
line of flight would be wise.
   I had to relocate my bees on short notice.