There aren't nearly as many horses in New York as there used to be, and there didn't used to be as many as there are now cars. According to this site the average life span of a horse in New York was only 2 1/2 years. During which time it produced 24 pounds of manure a day. Multiplied by 100,000-200,000 horses and you have enough manure to choke the life out of any city. Even before the advent of the internal combustion engine and automobiles, the horse was being replaced by steam powered trolley's in the cities and trains outside of them. A large city during the turn of the city was a very health place to live.

As much as I like horses, as a wide spread transportation system, their time has past. Leave them where they're at as animals that give us pleasure and occasionally do work that can't be done in any other way.