Maybe post horses were skinny and worn out, due to hard work and too little food? In the late 19th century the trams in Sweden were pulled by horses. I have heard that these poor animals were underfed and had a really ruff - and probably rather short - life. Post horses must have been something like that, donīt you think?

In comparison the horses of wealthy families would be matched in colour and size, well fed and properly groomed. The difference in appearence would probably be easy to see at a far distanse.