I know this is an oldish thread, but I wanted to come back and thank everyone for the excellent advice. I've gone into the paddock on a regular basis now with a whip and the other horses have learned to keep their distance. Today, I went in late in the afternoon (this is grain time for the horses so they want to come in), walked up to Harley, haltered him and walked him right between the other three who looked at me, but didn't move a muscle. I walked him right into the indoor arena (this is the usual route they take to be brought in for grain), calmly shut the gate behind me and again, they didn't even take a single step toward me.
I could not have done it without all of your help. THANKS! Oh, and oddly, the herd dynamics seem to have evolved. The old gelding who used to be the most aggressive towards Harley now ignores him. The big percheron X who is the alpha leaves him alone too, but oddly, now Harley seems to be challenging HIM! They will rear and Harley will take "air" bites at him. It's very odd. But I guess herd dynamics change. I don't think Harley would ever become herd leader, but then again, I never thought he'd challenge this big percheron X either! Maybe he's just trying to assert his place and I would assume that to be a positive thing.