In the early 90s (here in the UK at least) both styles of games were referred to as beat 'em ups. Proof of this can be found in issue 22 of the 1992 Mean Machines review of the SNES conversion of Street Fighter 2 in which they clearly refer to the game as a beat 'em up in the opening paragraph. Additionally, Computer + Video Games magazine did a whole mini mag special on "beat 'em ups" in their August 1992 edition which included both styles of game such as Double Dragon and Street Fighter 2. Back then, the two styles were differentiated by attaching a prefix of “scrolling” or “one-on-one” before the term "beat 'em up", and this worked nicely for many years. It makes sense when you think about it as it doesn't matter if you are facing one enemy or many - you are still beating people up.
Many other names have cropped up over the years for both sub-genres though. Brawlers, competition fighters, smack 'em ups, belt scrollers, and simply just fighters, but they all point to the same bruised and blood soaked gameplay. The only real difference being the amount of enemies that you face on screen and whether or not you move from left to right or are contained in a confined space. In short, call them what you want, but don't be raging at anyone for saying what you think is the "wrong" thing.
Karate Champ was a one-on-one martial arts simulator developed by Technos Japan and published by Data East. Despite looking and sounding phenomenally "8-Bit", the game was actually rather complex in nature, and arguably the most accurate depiction of the martial art that has been made into game format. However, the setting is heavily geared towards the sport of karate rather than the grimy street self defence that the beat 'em up genre is know for. In the game you play a nameless Karateka who enters a competition and you face off against an opponent in red (sound familiar?). That's it! Karate Champ was unique in that it had two joysticks instead of a joystick and buttons so pulling and pushing combinations resulted in a selection of standard karate moves which seem to be very accurate to the martial art itself.
Karate Champ also differed from it's later one on one offspring in that there were no energy bars of any kind, it was a one hit one point or 'Ippon' deal and then on to the next round. This system made you choose your attacks cautiously instead of just mashing and hoping for the best as you didn't have energy bars to pad the round out. This added a acute sense of tension to the proceedings and also a certain amount of strategy that is not uncommon in a real karate match (as someone who has been in a few Karate competitions can testify to). Karate Champ also offered up some fun bonus stages in the form of bull dodging (???) block breaking and other fun breaks from the concentration.
The next phase in beat 'em up's evolution was also to come in 1984 when Irem released Kung-Fu Master in arcades - and the very first scrolling beat 'em up was born. Kung-Fu Master was a simple affair that was obviously based in part on Bruce Lee's concept for Game Of Death, but one that would shape fighters for years to come.
In the game your girlfriend is kidnapped - a plot device almost murdered by it's own overuse in the genre - and you have to go and rescue her by entering a temple with five floors, each guarded by a boss.
The localisation of the original Kunio-Kun game (which was based on teenage Japanese street gangs) meant some rather racist redrawing of sprites and changing of setting to appeal to American audiences who apparently had a problem with a Japanese hero
The first, most famous and arguably the best of these were Konami's Yie Ar Kung-Fu and Way Of The Exploding Fist by Melbourne House. Both released in 1985 for the 8-Bit home computer market (though Yie Ar Kung Fu was originally an arcade title it was arguably more famous for it's home computer incarnation).
Both games are one on one fighters with Way Of The Exploding Fist being little more than an excellent version of Karate Champ but was probably also the best (only?) martial arts game on the ZX Spectrum at that time. Yie Ar Kung-Fu took a further step towards actual combat with it's life meters and variety of moves as well as it's colourful graphics and character variety, which until this point hadn’t really been seen in a one on one game.
Striding forth from his lair at Castle Stareskull one morning, Prototron decided to not reign down terror on the villagers, but instead go back inside, crack open a beer and load up Streets Of Rage 2. One hundred years later, he's still there. A avid music maker (of TERROR!) and retro gamer, he can be found whooping any and all heroes at all manner of SNK-based challenges.