A prototype of an unreleased Game Boy Advance game called Hi Hi Puffy AmiYumi: Puffy Ami Yumi and the Manga Madman has recently been found and preserved by the preservation group Codebound.
Hi Hi Puffy AmiYumi, in case you're unaware, was a Cartoon Network show that ran for three seasons between 2004 to 2006. It focused on the adventures of the best friends, Ami and Yumi, who were fictionalized characters based on the real Japanese pop rock group Puffy AmiYumi.
During its run, the show spawned two video games — Hi Hi Puffy AmiYumi: Kaznapped! for Game Boy Advance and Hi Hi Puffy AmiYumi: The Genie and the Amp for the Nintendo DS — but unbeknownst to fans, almost had another that was abandoned for unknown reasons.
As far as we're aware, the existence of the game only came to light last week when Codebound published a prototype build online, with the help of an online user named MrPinball64. It seems to have been in development at Altron following the release of Kaznapped! and focuses on the main characters exploring a bunch of different manga worlds using a copier machine (similar to in the Season 3 episode Manga Madness Part II).
The prototype build is labelled "Milestone 3" and is from early in development. It includes two selectable characters with various outfits to pick from, in addition to 3 worlds (Mystery, Adventure, and Mythology) to explore. These worlds contain 9 platforming levels and 3 bosses in total, but are all incomplete with combat and environmental interactions being unfinished.
If you want to explore the build yourself, it is available here.