The PS3 version of the Atlus-developed game Catherine has an unused and unique arrangement of the Diamond and Pearl Wild Pokémon Battle theme in its game data. It is titled poke_btl_loop.
https://tcrf.net/Catherine_(PlayStation_3)#Unused_MusicSword and Shield's files include an early prototype image of Pokémon XY, seen here.
https://tcrf.net/Pok%C3%A9mon_Sword_and_Shield#Early_ScreenshotThere is an unused function in Ruby and Sapphire's code that would've allowed the player to combine two moves together into a single move in double battles. The function only had support for Gust and Ember, which would've combined into Heat Wave.
https://tcrf.net/Pok%C3%A9mon_Ruby_and_Sapphire#Unused_code_for_Move_CombiningThe '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 Pokémon 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 for 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 
