Would commuting on a horse be more eco-friendly? What if there were millions of people did that? Would millions of horses be better than millions of cars?
In large numbers, horses are more problematic than cars.
According to Eric Morris, in 1898 delegates from around the world gathered to discuss urban planning. The issue they were "desperate" to solve was what to do about horse manure.
Rutgers University has a fact sheet about horses and manure. It states that a 1000 lb horse will produce about 9.1 tons of manure a year. Additionally, each horse needs about 10 to 20 pounds of bedding each day. If you assume 15 pounds of bedding per day on average, that's 2.7 tons per year. Combine this with the amount of manure each horse produces and it's 11.8 tons of waste each year. This excludes horse urine not absorbed by bedding which adds a further waste burden to the environment.
One million horses will produce 11.8 million tons of solid waste each year. Disposing of that is a massive environmental problem and why the delegates to the 1898 conference in New York were desperate about the waste produced by horses. With so much manure to dispose, it ground water can easily become contaminated.
Additional problems produced by horses are what to do with the numerous quantities of horses that will die each year.
The removal of horses from urban environments has resulted in one of the major improvements in human health. The permanent removal of horse manure from urban environments has significantly reduced the amount of disease carrying flies in cities and towns. Also, people in cities and towns no longer have to deal with smell of horse manure and urine or avoid having to step in horse manure or urine as they walk around town.
The other aspect of horse manure is that horses need to eat. According the Eric Morris, each horse needs about 1.4 tons of oats and 2.4 tons of hay each year. That's 3.6 tons of food each year. Rutger's University uses a figure of 25 pounds of dry feed per horse each day; that's 4.56 tons per year. Growing that much food for horses will require a lot of land.
I think the main problem is that commuting is almost never "eco-friendly". There's a lot of energy that's needed to carry your behind (and your carriage) from your home to your workplace.
I think the most efficient way to commute it is to use public transport, for which the weight-of-vehicle to weight-of-passenger ratio is lowest. For trains, the amount of friction is also reduced to a minimum.
Horses have high friction, they need a lot of rest, and they keep "running" (i.e. breathing, eating, etc.) even if they're not carrying anything.
I think it is a partial misconception that horses are a good method of transport. In old Asia, messages were sometimes delivered by "marathon monks", messengers who were running on foot because they turned out to be faster than conventional horse-riding messengers, because they could run for longer and needed not as much rest. Horses overheat much easier than humans. In fact, there are tribes in Africa to this day, who hunt (in groups of three) by running behind antelopes until the antelope collapses from exhaustion. (And that's without anyone riding the antelope.)
So, if anyone should run you back and forth between your home and your workplace, it should be you. (I guess the distance and the size of your briefcase is a limiting factor here.) =)