To settle your own conscience, this guy was 100% in the wrong. You just don't go throwing a strange horse in with a herd without biting and kicking going on. Odds are someone is going to get hurt, you are just lucky none of yours came up hurt.
As someone else said, this would go to small claims court. In that case round up all your horsey friends, print up a few articles about how to introduce horses, etc. Take them along with you to court just in case the judge doesn't know horses. Also, watch some Judge Judy so you know the does and don'ts. Those who go most prepared more often then not win.