Tires, metals, lubricants, plastics.. How is a bike more environmentally friendly? Many bikes are made in China, so you have shipping and transport as well. Have you ever visited a bike factory in China? How about a tire factory? A horse ranch is infinitely more environmentally friendly.
A horse emits an average of two tonnes of carbon emissions a year [1], comparable to that of a car [2].
The full picture gets complicated when you account for vehicle fuel, horse diet, age of vehicle/horse, land use, storage etc. But in general it's fair to say that horses are broadly similar to cars in carbon impact.
Compare both with pedal bicycles, where impacts are effectively negligible [3]
A car only emitting as much as a horse makes car emissions seem quite good. The issue seems to be the difference in where the two source their emissions from: oil vs plant.
