Horses are quite inefficient; in fact, all animals are. The only reason it's cheaper to ride a bicycle someplace than a motorcycle is because of the lesser weight and speeds; if you created electricity with a human-powered generator like [this](http://www.los-gatos.ca.us/davidbu/pedgen.html), you'd actually burn more energy in extra food than you would by getting the same amount of electricity from a gasoline-powered generator.

...which, of course, isn't a direct concern since many people bicycle for their health anyway. But it is a concern when it comes to more equine suggestions.