This question came from an obscure, throw-away sentence in a Terry Pratchett novel...
Imagine, if you will, a bicycle with no pedals, chain, or self-propulsion mechanism at all. A person sits on it and can steer, but that's it.
Instead, it will be pulled forward by a horse.
Could this bicycle actually move forward, allowing the rider to balance? Or would it be impossible for the horse to get it up to speed before it falls over?