Bullgine is, indeed a term applied a small steam loco- motive which runs on short tracks and is usually used to haul ships through locks and canals. A donkey engine is a stationary engine which might be used on board ship to power a winch or to run a pump; it has no tracks, tending to be portable, if not mobile. A pony engine is a similar small locomotive built to use regulation-gauge railroad rails, and used for moving cars in a railroad switchyard. The earliest entry I could find for the word in a quick search was 1846; it was in use as late as 1939.