I've heard of this game before, I remember looking it up quite a while ago on Wikipedia after coming across it somewhere. I don't know much about it however. Seems like it's one of those obscure monster raising series that was never released overseas. If there's no english translation available there's not much you can do really (except learning to read Japanese ^^;). You won't be able to play the Japanese game on an overseas console since it will be made for Japanese N64's (I think, unless the N64 is region free?).
I love seeing new monster series that never made it out of Japan, it's actually surprising at how many were made.
EDIT: Your post just reminded me of the other monster series I found on the day I looked up Onegai Monsters. One of them was Gramon Battle (
http://en.wikipedia.org/wiki/Tensai_Bit-Kun:_Gramon_Battle). There was another one where you drew your own monsters and battled them, but I can't remember the name at the moment ^^;
EDIT2: I remembered what they were called. I don't know if they were monster RPG's specifically but here they are: Magic Pengel (
http://en.wikipedia.org/wiki/Magic_Pengel) & Graffiti Kingdom (
http://en.wikipedia.org/wiki/Graffiti_Kingdom)