If you want to become a "horse rescue" then you've just taken the first step.
For background, I was the VP and Large Animal Committee for our local humane society when this happened to a friend of mine. We did as I suggested (called the sheriff and made a report; AC ultimately identified the owner); the group took legal custody of the horse; I ended up doing the rehab; the owner paid for about half that cost; we sold the horse and recouped the balance of rehab costs. It worked out nicely at the end for everybody (including the owner who was not charged with abandonment). You're predicament is on all-fours with ours. Do what we did.