Creosote poles cut into lumber is being used by many here.
The one problem with creosote and horses is that, if they get a splinter, creosote will cause a huge abscess.
I have heard of several horses that scratched their shoulder or behind and then had such a terrible abscess that took months to heal.
One a friend’s cutting mare, that the vets finally went in there surgically and found a tiny sliver still left in there.
Once out of there, she finally healed, but it left a hole in the muscle of her back end.
Her vets said if it had been mere wood, she may not even have a abscess from it, but creosote soaked wood is very caustic inside the muscle.
We have painted plywood and barn framing wood with creosote over years, where horse may reach to chew on it and they leave it alone.
That never caused any problems.
Wood that is treated all thru with creosote and that may splinter somewhere in a horse, like boards in horse stalls and pens?
I would think about it before using it in too many places.
Also, those boards are very expensive, compared with other kinds of fencing, because they are considered very pretty and in horse pens, horses can’t get to each other to bite and so have less stress and injuries.
Creosote treated railroad ties or telephone or high line poles for fence posts should be fine.
There are trade-offs to all you do.
With all the information you have, you can decide what you want to use that will work best for you.
