According to Hideo Yoshizawa, some concepts of the game, such as the camera system, were previously conceived for a game based on the manga series “Spriggan” that was cancelled after only a month of development.
Concept art exists of an Octopus Moo which was scrapped. Despite this, it can be seen frozen in a stalagmite in Vision 4-2
An illustration of a pink & green Klonoa, who wears a cap of Blinky chasing Pac-Man, can be found in the original website for Door to Phantomile around the developers' section. It's possible that it may be related to the scrapped idea of Huepow betraying Klonoa and could be the inspiration for the doppelganger in Shifty Look's "Klonoa: Traveler of Noctis Sol".
There is art in Namco's Japanese Door to Phantomile pages that show official illustrations of Klonoa going against Huepow. Loose translations state that Klonoa would have to use his own soul to power the wind ring in this battle.
Pushing L2 and R2 on the title screen will blow the leaves in the background away.
Door to Phantomile began as a much darker game involving robotic stone-like figures. But, due to most games in the Playstation era already having a dark theme, series creator Hideo Yoshizawa decided to make it into a more lighthearted game while still containing a sort of dark undertone to it. This later became the defining traits of the Klonoa series.