So this is lost on westerners but the early games were actually the first real attempt to turn the manga into a legitimate game...in Japan those weird rules came before the game as we know it was solidified in Battle City and the physical release of the OCG.
In those games you can fuse monsters without polymerization? A lot of early Fusion monsters debuted in those games and one of their possible Fusion combinations (as each Fusion had multiple ways to fuse into it) was chosen for the actual card game.
Of course in America we got the real card game first.
Yep, without polymerization. And as you wrote, you didn't need specific monsters for the fusion. I looked it up, it's been called "Forbidden Memories". It's been super hard to be honest, but felt very rewarding when you finally got a proper card after a battle!