The PS3 version of the Atlus-developed game Catherine has an unused and unique remix of the Diamond and Pearl wild Pokemon battle theme in its game data. It is titled poke_btl_loop.
https://tcrf.net/Catherine_(PlayStation_3)#Unused_MusicThe disassembly of Pokemon Red confirms Morimoto's story of shoving Mew in at the last minute: Mew's data was included at the beginning of the main assembly file, away from where all other Pokemon data is included.
https://github.com/pret/pokered/blob/master/main.asm#L23-L25The filename for Hau's battle theme in Sun and Moon is bgm_nj_vs_friend.dspadpcm.bcstm. Likewise, the filename for Gladion's battle theme in Sun and Moon is bgm_nj_vs_rival.dspadpcm.bcstm.
Sword and Shield's files include an early prototype image of Pokémon XY, seen here.
https://tcrf.net/Pok%C3%A9mon_Sword_and_Shield#Early_ScreenshotThe '97 Spaceworld Prototype of Gold and Silver had a unique send out animation for Hoothoot. Discovering this led to the discovery that said animation still existed in the normal Gold and Silver ROMs, albeit buggy and unused; verifying the legitimacy of the Spaceworld ROM.
https://tcrf.net/Pok%C3%A9mon_Gold_and_Silver#Unused_.E2.80.9CSend_out.E2.80.9D_AnimationsThere is an unused title in Pokémon Crystal for Pokemon in the PC that have entered the Hall of Fame over 200 times, called "HOF Master!" It is unused due to a design oversight.
https://twitter.com/crystal_rby/status/934800226883338240 https://github.com/pret/pokecrystal/blob/master/docs/bugs_and_glitches.md#a-hof-master-title-for-200-time-famers-is-defined-but-inaccessibleThe map illustrations in Gen 3 and 4 were not done by Ken Sugimori, but were done by an artist known as Midori Harada.
https://bulbapedia.bulbagarden.net/wiki/Midori_Harada 