Also notice how many pokedex entries mention animals like Pikachu the mouse pokemon implying normal mouses exist.
i always tought it was something like noticing something recurring I. E. glameow skitty and purrloin have similar shapes and maybe it became known as a cat shape. therefore cat pokemon. i do not know if those are their actual classifications in the pokedéx.
the thing that bothers me the most is ghastly's and raichu's comparison to indian elephants hinting that kanto and co. are other places in our world. although it is more probable that those are metaphors to explain to players in terms we can understand