Bicycles and walking are powered by very non-green energy sources. An engine that spews CO2 from one end and methane from the other end.
I don't know if you're being funny or serious, but that's a common misconception so I'll assume you're being serious. The CO2 and methane that we animals spew all came directly from the atomsphere through the food chain or through breathing. It's a closed cycle - plants and animals take CO2 from the atmosphere, store it a short while, then release it back into the atmosphere. That closed cycle is the gold standard of sustainability, pretty much the opposite of "non-green".