Did Pokemon start off as a Manga? Was Pokemon initially manga and then tv producers changed the story they wanted it or was it a product of imagination of tv producers?
Pokémon actually started as a pair of video games published by Nintendo for the Game Boy in 1996. Known as Pokémon Red and Blue (although in Japan the games were released as Pokémon Red and Green), these games were followed up by some short stories.

IIRC, first came the Pokémon comics, then the Pokémon series debuted on television on April 1, 1997, and shortly thereafter the manga was released on August 8, 1997.