Why does this game have a ESRB rating when it was released a full year before the ESRB exitsted ? From looking at the cover art I can't even find a RSAC rating.
Yup, it must be from a re-release. If I recall correctly, and I'm almost positive I do, the game had a "voluntary" rating, much like the PC-13 rating for Wolfenstein 3D.
Here, take a look at the 3D Realms website. Scroll down and they show the game as having an M rating, although as you correctly pointed out the ESRB didn't exist at the time of the game's release.
Then shouldn't there at least be one box or something showing the rating ?