The correct answer is some Japan-only release that sold like absolute shit.
Yeah, this is the real answer. Something that never left Japan and is obscure even there. The PS1, PS2 and even the PSP have a lot of titles fitting this category
With all that said, it's not interesting to drop an answer like
this or
this or
this. So I'm gonna recommend some forgotten titles that non-Japanese gamers can get something out of:
Poinie's Poin
I know what I just said about Japan-only titles, but this is worth bringing up since it's actually fully playable in English. Yes, despite not releasing in any English speaking regions, this PS2 game has a full English dub. The game itself also has a really, out there aesthetic too (I mean just read the dialogue on that thumbnail)
OreShika: Tainted Bloodlines
It's not the most obscure title posted in this thread, but it's still the Vita only sequel to a Japan-only Portable remake of a Japan-only PS1 JRPG, so it's quite niche... it's also still stuck on the PS Vita (why Sony, why did you not port this and the other Vita exclusives to the PS4). It's a shame it's not more known since it has a very unique gameplay conceit (you control a cursed, fast ageing family / clan, and you're basically fighting a villain over several generations)
The Yarudora series
This is the real obscure shit. So the Yarudora series is a series of animated visual novels and uh... that's basically it? They're not sequential, only really sharing a theme of amnesiac heroines. They really are just visual novels though the production values (with animated sequences and even anime-style openings) are impressive. These Japan-only games originally released on the PS1 and later got re-released on the PSP. Why am I mentioning this? Apparently the first game in the series, Double Cast, got a very recent fan-translation.The translation patch seems to be specifically for the PSP version but basically, it's now playable in English. I can't say much of the plot, but I know that it definitely *goes places*. I don't know if all the games are getting this treatment, but
there is a subtitled playthrough of one route in the fourth game on YouTube.
Honorable Mention: Koukidou Gunparade March for the PS1. This game seems to be legit, but the only thing non-Japanese audiences got was the anime adaptation, so the series outside of Japan is remembered only for that