I would asume though, that the bill is because the carriage
horses stand for sometimes long periods
of time. Usually a horse out in pasture in the middle
of winter would be walking around and foraging for food which keeps them warmer than simply standing and not eating at all. I would say that the only time they would need to be blanketed is when it is snowing that wet heavy snow and is windy because then they could get a chill easier.