Today I found out what exactly a pony truss is. I had heard the term before, but never understood what it meant. In fact, it is quite simple. A pony truss is a truss bridge which allows traffic through the truss, but the top of the bridge is not joined together with cross braces.
Example of a real pony truss bridge:
Any truss bridge can be a pony truss. But not many are being made anymore, so chances are small that you will actually see one.