I found an interesting example a few years ago called Trampoline Terror. It was published in the U.S. alone as far as I know, by Dreamworks for the Sega Genesis. Anyway, the NCS Masaya logo is what immediately caught my attention.
After analyzing the game more thoroughly just recently, I noticed some similarities between it and Mamono Hunter Yohko (some aspects of the presentation, and the dodgy game design :)), so most likely it was developed by KLON (the company behind MHY). A friend of mine just confirmed that the music is similar to that of Atsuhiro Motoyama's older soundtracks, who apparently worked for KLON before he started composing for Raizing. Bingo!
I still haven't been able to find a Japanese release yet, and neither have the lot of Japanese game freaks I've asked about this. Most likely, it doesn't exist.
In-game footage: http://youtube.com/watch?v=s38UuG8-7Jc
If you know other examples of JP games only released in the west, please post about them in this thread!