A lot of people assume that the Pokémon games are for kids because they tend to be popular with that demographic, but that's not necessarily the case. From the sounds of it, the first Pokémon was developed largely around the personal interests of the series creator Satoishi "Dr. Bug" Taijiri, who loved bug-catching, videogames, and rubber monster TV shows like Ultra Seven. Put them all together and you have Pokémon, or as it was originally called, "Capsule Monsters." The name had to be changed due to copyright issues.
If you want to know more about the origins of the series, check out the feature over at Glitterberri. They've pulled some art and translated some text from the Japanese book Satoshi Tajiri: The Man who Created Pokémon for a little feature that may teach you a thing or two. Pokémon used to be near-Godzilla sized, and trainers used to punch them, or even whip them! That's just a start of the differences between Pokémon and Capsule Monsters.
I wonder if the series would have gone on to be as popular if stayed true to the original vision?
Pokémon Early Concept Art [GlitterBerri's Game Translations]