One thing that has worked great for me in this situation, in all instances I've used it in is this:
Put one lead with a chain over the nose on the horse, and a second regular lead rope on the bottom of the halter on the regular ring.
Do you have feed set out for the horse prior to bringing it into the paddock? Hay even? If so, stop pre-feeding this horse forever. Make the horse know that you control the feed. This could be one reason that the horse thinks it needs to race off-- to get food.
Back to the dual lead lines: In this situation you'll first take the horse into the paddock JUST to be in there, and then take the horse out right away. Have someone hold the gate for you so you can exit smoothly. Walk in, walk out. Do this several times.
When the horse can do this smoothly, walk in, halt for a few seconds, and walk out, same thing. You are working towards the horse standing calmly and assuming it'll be taken directly out again. Use the regular (non chained) lead as the primary, and the chained lead for reinforcement and to bring attention back to you.
When you get to the point where you are going to practice releasing the horse, hold firmly onto the chained lead (do NOT wrap it around your arm or leg or body under any circumstances) and you'll be unhooking the regular lead first. I suggest wearing gloves. Here is the process:
1. Walk the horse into the paddock, and make a small circle so that your back is to the gate, and the horse is facing you and the gate. You need to have plenty of space to get the heck out of the way if the horse pulls a fast one and spins around to take off bucking. Chances of this are very good-- and honestly that is alright... as long as the horse learns that it needs to stand still for release.
2. Tell the horse whoa. Ask him to stand for a second. He'll expect you to walk back out the gate since you've practiced it a thousand times. Instead, reach your hand up to release the normal lead. Be prepared for the horse to pull and fly, and the second he tries, give the horse a huge crack with the lead over his nose. It'll startle him and make him stop for a minute. Yell whoa, etc. Walk back out of the paddock. You have to be very fast, and anticipate his movements.
3. Rinse, repeat. It will take diligent practice over many days (possibly weeks), but it can be overcome, even for someone as little as you.
Obviously if he has learned to stand still for undoing the regular lead, and he's fine, let him go. To do this, I'd re attach the plain lead, and take off the chain first, and then release the plain lead.
Owner and head trainer of SE-Wisconsin Horse Care
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