I realized the ALTERED BEAST arcade was released same year as SH (1988)

I can see a certain relation there...

Hero starts being a sort of "normal guy", then... dies? And then the game starts and he's thrown in a sort of "hellish place" where he must fight rotten zombies.

Also, from the "normal guy" from the start, he received an external power given by MASK/LIGHT BALLS wich transforms his body into an ultramuscled one.

So, could one of these two games inspired/influenced the other? (No idea wich came out first though) ... (Also, no idea if they could share common creators)

Chances are they probably aren't related. I'm pretty sure both games were developed by in-house teams at both Namco and Sega (Team Shinobi was responsible for Altered Beast, IIRC), and I don't believe there was any cross-pollination between the two.

However, there are definitely similarities. In fact, I'd planned on doing a "similar games" feature on West Mansion for a while, but never got around to it. Altered Beast was on the list, as was Monster Party.