The idea that Pokémon is a contraction of the English “Pocket Monsters” is blatantly untrue when put under any kind of scrutiny; the poke element is a Japanese rendering of Pokkim (”Undermountain”), a term originating from the Ainu dialect of Sinnoh, who believed that the creatures spawned from the depths of Tengansan (Mt. Coronet). It is due to this primordial connection that the Ainu held rock and ground pokémon in particularly high esteem, specifically Rhyhorn, who they honored as the “first” pokemon. It is only with the later settlement by the Yamato people that we see the cultural ascendance of “heavenly” pokemon - Dragon, Flying, Fire, Electric, and Water.
In the Kofun period, as the early Japanese courts adopted Chinese culture, plant-type pokemon rose to social prominence, as Chinese philosophers glorified plant-type pokémon for their value in cultivating land, sustaining civilization, and promoting social harmony with their aesthetics (and, as would be later understood, chemical manipulation of the human brain). For this reason, the bulbasaur, a ubiquitous pokemon in China, was adopted as the chief pokemon of the Japanese imperial courts, despite the creature being alien to Japan. Later Japanese emperors would enshrine the mountain-carving Korean Blastoise and, much later, the European Charizard as equal symbols of Japanese cultural reach, but Bulbasaur always held the position as the primal pokemon of Kanto.
This “plantmania” did have a literally toxic knock-on effect, as the Kanto population of plant-type pokémon are entirely poisonous, unlike those found in other regions (including the imported Sinnohan Tangela; note that Tangela only dwells in a patch of grass near the aging port of Pallet Town, and in the Safari Zone). This led to a disproportionate amount of power concentrating in the hands of the psychic-training mediums in Kanto society, including the notorious “shadow regime” of the Itako of Mt. Osore.
Due to the psychic stranglehold on Kantese politics, the people were unusually ignorant of Ghost-types (with their existence only confirmed with the Sylph Co.’s work at the end of the 20th century), and Dark-types had been completely exterminated from the region despite their presence in neighboring Johto (with the dark variants of Kantese pokémon establishing roots in Pacific Island chains such as Alola).
Oda Nobunaga introduced Steel and Dark-type pokémon into the region in his attempt to unify Honshu and wrestle power from regional psychic priesthoods; While these types remained in Johto, their population never stabilized in Kanto and faded quickly after Nobunaga’s death in Honnō-ji at the hands of Akechi Mitsuhide‘s Portuguese Charizard Kapadokya.
At the end of the Kofun period, the Emperor established a rule to curb similar excesses; the peasantry could only own normal and bug Pokemon, while those of the merchant class could use water and flying, and no more than three. The other types, especially the “heavenly,” were the domain of the nobility; a noble could train up to six pokemon, but they must be of the type of their house. The Dragon-class was the sole domain of the Emperor and his household, who alone could raise pokémon of any type, without restriction. The Dragon-class privilege was later extended to the shogun in the Tokugawa period, leading to the eradication of the Fairy-type from Japan, with most local varieties of those species bred into normal-types.
This legally enforced connection of the poke to the symbolism of the noble and imperial houses lead to their conceptual merger with the Japanese system of heraldry, the mon.
In the Kamakura period, Johto became a major cultural and commercial center, as its rich soil allowed the production of Apricorns, which could be ritually altered with carving and mercury inlaying to seal away pokemon in a small, weightless space. These sealing spheres allowed the development of a far more efficient and mobile pokémon warrior class, leading to the ascension of the pokésamurai and a strong ronin underbelly, with Minamoto no Yoritomo generally acknowledged as the first pokemon master.
In the Meiji period, these noble houses’ type-monopolies were disposed of, and pokémon training was reorganized into the Gymnasium system of continental European origin.