I'm probably wrong, but I'd say Kung Fu Master was the first scrolling beat-'em-up to arrive on the scene, and possibly even the first conversion of an arcade beat-'em-up. It's also absolutely terrible (an no question there).
Although the arcade version was very popular, despite its rather restriced gameplay, US Gold really fowled up with the conversion. The graphics are absolutely chronic, with colour-clash everywhere, and the whole thing plays as if it's underwater. There's also the diabolical music to worry about.
However, Kung Fu Master does contain a number of milestones. Apart from the scrolling, it also introduced the idea of multiple opponents, some of whom are armed with nasty weapons, and the end-of-level guardians can still cause some hassle.