that fencing you see around building sites, large panels of small metal mesh about 8' high with heavy feet to slot into, is very cheap to hire. i think it's called Harris fencing or Rhino fencing. i'd use that, i think. no horse on earth can jump out of it, it might need a bit of attention from a hammer if there's any sharp bits, but i'd use that if i needed to pen one in... or i'd make a pen out of elec fence and tall posts.