I'm taking delivery on one of these in 2 weeks.  I'm really excited, it seems to be the safest corral option for the type of camping I do (crowded Endurance ridecamps).  www.ciwtrailers.com/safetypen.htm 

Yes, it's expensive, but it's comparable in price to other commercially-available portable panels.  And I'm sure there are other folks on here who want something that is easy to set up, lightweight, is tall enough to discourage jumping out, and looks solid enough to discourage outside loose horses from running through.  (At every ride I've been to, there has been at least one horse get loose from an electric string pen, and usually they run right through someone ELSE's electric string pen, taking that horse with them.)  You can even get electric wire along the top and bottom to prevent leaning.

I do not work for the company, and am not receiving anything in exchange for my recommendation.

I looked into them and they are very expensive.   I have looked at other options as well.  I seen one gal use snow fencing (the plastic kind).  She ran that around posts and then ran a line of hot wire on top.  Deer didn't run through it and the horses stayed inside safe and sound.  Compact, lightweight, and easy to assemble.  I am going to look at panels at Tractor Supply.