This horse has learned to intimidate people. Do NOT enter the pasture without a lunge whip, ever, and be prepared to deliver a hard what on his neck to turn him away. And I mean hard. Another horse would bite hard or turn and plant both heels on whatever body part was closest if another horse came into his space uninvited. He may run off. Good, your message was loud and clear. His space is beyond the reach of your arm so get tough with him if he comes in to it. You can go in to his space but he must learn to never enter yours. Be fair and don't whack him if he stays 3' away. That distance is a circle all around you so he's not to invade it from behind either. When you put him in the round pen, it's not a place to chase him in endless circles. 3 circles in each direction are plenty but it's the reverse of direction that wins his respect as you are telling him to move his feet where you want them to go, not him. When your satisfied with even small increments, step back a few steps and see if he will look at you with both eyes. That means his attention is on you. If not, then move him again. I'm going to suggest you get the ground work in place before anyone rides him again. Get the respect first and not turn it into a rodeo.