I suggest you paint the posts with creosote as no matter what you apply to them the horse will nibble but with creosote they won't eat the posts. It will make the poles a brownish colour. It does smell awful though so be prepared to be overwhelmed with it and wear something you can throw out after as it stains like the dickens. Also check environmental regulations on it for your area. I think the one used on wood is referred to as Coal Tar Creosote. I did a whole barn in it when I was younger for my employer. Awful stuff but very effective. Their may be other products now on the market as well. Check with your local feed mill too!
"For every animal that dies in a shelter, there is someone somewhere responsible for its death".