If it can actually stand on the road then try the police, as a car could hit it and it could cause an accident.
It is currently not against the law to chain a horse like this so the RSPCA cannot really do anything.
When the new Animal Welfare act comes into force they will be allowed to prosecute when an animal is likely to suffer.
Try and see if it has access to water, if not try giving it some in a bucket, but I would not leave the bucket with it as the owner will then think that someone else is doing his job and not bother.
There was horse near where I walk my dog, chained in a similar way, it had gone round and round the chain and ended up where it could not raise its head more than two foot off the ground, I spent half an hour trying to release it! I managed in the end, but I really despair of these people, how can they be allowed to own such a fine animal!!! I dearly love horses, but would never buy one because I do not have the facilities to look after it to the standard I want.
Good luck, sadly all you can do is report it to the police as likely to cause an accident.