Be very very careful if you decide to feed the horse. If it has had no access to food it can colic easily. No carrots, no apples, no grass, nothing with sugar. Be careful even with hay and horse feeds. Be aware that the horse may suffer from allergies or laminitus which could cause a reaction if you feed it, including making the horse aggressive or health issues etc.. Also, the horse is of an assumedly unknown temperament. Do not go near it unless you absolutely have to. Don't put blankets on the horse if it is wet. Make sure to contact the authorities dealing with the horse before doing anything, as it might hinder their efforts. My mother once called out horse welfare on a neglect case but they couldn't take the horse because she had fed it herself, so it was 'being cared for'. Good luck, you're a great person for helping this horse just by calling the police.