Pokettomonsutâ or Poketto monsutâ?

Pocket Monsters (Pokémon) in Japanese is "ポケットモンスター". According to IMDb rules, it should be entered in Hepburn instead of simply "Pocket Monsters". However, the Japanese name is the same in Pokémon and Pokémon Horizons, but the "original" (Hepburn) names are different on both pages.

Which is correct? I believe it's "Poketto monsutâ," which keeps it as two words, as was the original intention, but I admit that Hepburn as a whole doesn't make much sense to me, so I'm not sure if there's any rule about keeping the title together.