Make 2 strands hot, ground the others...T posts are fine...
Horses are funny...you can put one in the most smooth set of pens you ever saw and come out in the morning to see a gash on the horse.
Also, I've seen people keep horses in pens with wire, loose boards, all sorts of sharp and dangerous things...and they never get hurt.
When a cow gets into a jam, she'll stand there and slowly move her legs around to get out of a tangle or tight spot. Often, a horse will simply go crazy and run thru/over stuff that they know is there...but they got boogered for some reason. Net, the planned smooth wire fence will work for the horses UNLESS they get boogered just wrong, then it's good to have the smooth wire fence...barb wire just cuts them up ... doesn't particularly deter them when one has gone crazy.
True story...had an uncle who was around and trained horses all his life...roping, cutting, reining, ranch work, colts, studs, mares...you name it. He got an older gelding (not unusual) to break and train...two months into the training he was behaving reasonable, easy to saddle, get on, getting a good rein on him. Training was done in an arena with sand, fence made of V net wire...the kind you can't climb...large cedar (very visible) posts with cedar post railing on top of wire between each post. Fence right at 6 feet high. Point is the fence was very well built and visible and horse knew it was there....hours of training spent in the arena.
One day, uncle was taking horse in gentle figure eight pattern, loping. Horse came out of one loop, but failed to somehow change leads and enter the next loop, just "cold jawed" and ran straight at fence, completely unresponsive to the reins. Uncle perceived problem and threw his weight left then right in an attempt to throw horse off balance and regain control/sanity. Didn't happen in about 7 strides. Realizing fence impact was imminent, uncle loosened up in the saddle and went over the top rail as the horse hit the fence. Uncle got up and dusted himself off, looked around. Wire net was split top to bottom just like it had been cut with a knife. Horse laying there outside the arena, neck broken, dying. There is a reason there are the warning signs required anywhere there are equine events...it can be dangerous.