It is interesting to hear all the recommendations on fencing. It varies somewhat but the general principles are similar. It sort of depends on location, weather, purpose, how much work involved and cost. Here in Nebr., the ground is relatively rock free and drier than many other locations. For cattle, I use 5" wood creosote posts, augered in the ground and tamped in place. 6" or larger posts, braced in the corners and for longer runs, posts braced in the fence row. Occasionally, will use alternating wood and steel T posts. Creosote posts in our area will last 30-40+ years. Cattle can be abusive and push on fences even with barb wire. Horses will certainly push on smooth wire fences but an electric wire keeps the horses off the fence but the electric fence needs to be charged and maintained.
T posts have the potential to cut/injure horses much more than wood posts due to sharp top end. I used to be a veternarian and have treated my share of horse injuries due to T posts. I would not use T posts for an enclosure or smaller pen with horses.