What? Riding a bicycle is signifigantly more energy efficient per distance traveled. There is no way you produce more Co2 if you use a bike to cover a given distance than you do if you walk. And a healthy human is more energy efficient for long distance travel than a horse is, so the same concept would apply.
As for materials? You think saddles appear out of thin air? A full set of horse riding equipment is going to set you back a thousand dollars easy, and that is probably on the cheap end. You will need saddle, bridle, reins, saddle blanket and likely other things i am forgetting. Those have a production cost and maintenance cost as well. Hell, a full set of horseshoes alone probably contain more metal than my bike does.
Meanwhile, you can get a high quality general use bike for around 500 bucks. A well made properly maintained bicycle probably costs you less in maintenance than a horse would, especially considering you don't need to feed a bike. And oil? Lol. You probably use about as much oil oiling a pair of leather riding chaps as you would oiling a bike chain / gears in a year.
Also, you don't need to make bikes entirely out of metal. Composite Laminate Wood Fiber and Bamboo can be used to make bike frames that are environmentally friendly and just as durable as anything made of modern metals. Helll, these guys will even teach you how to build a bamboo bike frame from the comfort of your own home.