Due to the popularity of Pokemon in the late 90s and early 2000s, many pirated video games were made starring various Pokemon. These include games like Pocket Monster and others. But, there were also many hacked games made to include Pokemon characters replacing the original characters. These barely changed any aspects, if any, from the original games they were trying to pass off as Pokemon games.
NES Hacks
Most Pokemon hacks were released on the NES or Famicom. These include:
Pokemon Golden
Pokemon Golden (AKA Pokemon Gold) is a hack of Tom and Jerry on NES. The only difference is that some of Jerry's graphics are replaced with the Pokemon Eevee.
Pokemon Yellow
There are 2 different hacks under the name Pokemon Yellow. The first is a hack of Tiny Toon Adventures where some of Buster Bunny's graphics are replaced with a badly drawn Pikachu. The other game under this title is a hack of Felix the Cat where Felix is replaced with a gray colored Pikachu. The latter version is sometimes called Pokemon Silver due to Pikachu being gray.
(Felix the Cat hack)
(Tiny Toons hack)