Official English Taiko no Tatsujin use Inner Oni for Ura Oni (I'm pretty sure, but couldn't get any source to confirm).
I just looked up "Hell Oni" in osu!, and I only have 1 map with that difficulty name (http://osu.ppy.sh/b/102527). Maybe I'm just not downloading taiko maps enough.
In order of the most to least used difficulty name: Inner Oni, Ura Oni, then Hell Oni.
All the new ones I download uses Inner Oni instead of Ura Oni, so there's probably a rule for taiko mapping to use Inner Oni instead now.
I just looked up "Hell Oni" in osu!, and I only have 1 map with that difficulty name (http://osu.ppy.sh/b/102527). Maybe I'm just not downloading taiko maps enough.
In order of the most to least used difficulty name: Inner Oni, Ura Oni, then Hell Oni.
All the new ones I download uses Inner Oni instead of Ura Oni, so there's probably a rule for taiko mapping to use Inner Oni instead now.