Pokémon is the most profitable franchise in the world. From video games to cards to movies, the humble pocket monsters represent one of the widest-reaching franchises in the world. Even through all of this, Pokémon has managed to remain mostly bright and cheery, generally revising dour corners of its canon into happier stories and ideas.
This isn't to say that fans haven't tried to take Pokémon down a darker road. Many have pointed out inconsistencies in the canon of the show or video games that make the characters somehow deeply morbid, such as the prevalent theory of Ash being in a coma. A recent fan theory, aimed more toward the video games, asks why Pikachu can be found in their native home, with the solution bringing the real-world plague of animal abandonment into play.
Reddit user u/Vexelius has taken two major indicators from Pokémon canon to theorize why Pikachu are so rare in the first games, as they are only found in Viridian Forest (and are rare in this area, too). According to the theory, Pikachu aren't native to Viridian Forest. In fact, they were abandoned there en masse following a pet craze that occurred before the start of the games.
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Shockingly, there is support for this theory in canon. As indicated in Mimikyu's Pokédex entry in Pokémon Sun and Moon, the creepy fairy took on the appearance of Pikachu because of the mascot's prominence some 20 years previously, indicating that Pikachu's increased popularity was felt worldwide. While this extreme popularity is naturally a reference to the sudden worldwide popularity of the Pokémon franchise, this spike in popularity has never been established within the world of Pokémon.
The fan theory suggests that this spike in popularity took place shortly before the first games, ebbing before the protagonist started on their journey. This would allow for the 20-year timeframe to be met. However, the lack of Pikachu elsewhere in the games beyond Viridian Forest implies that the electric mice were disposed of. The canon maze-like nature of Viridian Forest served to make it a perfect location to dump an unwanted Pokémon that owners never wanted to see again. Thus, the Pikachu encountered by players in Viridian Forest are abandoned Pokémon or their descendants.
This theory not only paints generally responsible and kind Pokémon trainers as subject to the whims of popular culture but also suggests that the issue of pet abandonment in real life transfers over to the world of Pokémon. The increased sentience of the powerful creatures makes their abandonment just that much crueler. If Pokémon trainers are subject to short-sighted whims and fads, then their Pokémon could suffer, as could the environment. The invasive Pikachu species may well lead to excessive competition and death or adaptation of other Pokémon, echoing conditions that led to the likes of Cursola and Hisuian Zorua/Zoroark. This fan theory is terrifying in that it works so well within the canon of Pokémon but threatens to expose malevolent forces within the generally bright and happy world.