Pokemon was based off of a franchise called Pocket Monsters.
Digimon was based of a franchise called digital pets.
Then they both started to have a tv series and a video games series. So, no one copied anyone.
They're both different because pokemon, you can catch and tame many pokemon as your partners and defeat gym leaders. In digimon, you're a digidestined, getting a crest, a digivice, and only 1 digimon by your side.
( I only went into explaining this all because I researched it al last month and I LOVE both series, so I just wanted to get that straight, I guess.)