OK: a very early metal airplane built by Junkers was nicknamed the "Tin Donkey" and one of the world's first successful airliners was the Ford Tri-motor, known better as "the Tin Goose." A German nickname for the bicycle is Drahtesel, or "Wire Donkey," not to be confused with the old Mandarin word Tiema, or "Iron Horse." But I am particularly fond of the donkey connotation when I think of Robert Louis Stevenson's book, Travels with a Donkey, where he says:
But we are all travellers in what John Bunyan calls the wilderness of this world--all, too, travellers with a donkey; and the best we find in our travels is an honest friend. He is a fortunate voyager who finds many.