Put out found ads on Craig's list, and in the local paper, or local radio stations. However, when you put the adds up, DO NOT give a description of the horse, simply say "horse found, call *insert number here* for more information". When someone calls, ask for a description of the horse they are searching for, and make sure they describe the correct horse. This will prevent some random stranger from being like "oh yes, I have been looking for poor scooter!" and then selling "their" horse for dog meat. If no one claims him after a few weeks, I guess you got a horse!
"two things are infinite: the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein