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.
Most Pokemon hacks were released on the NES or Famicom. Due to suitable graphics of the characters not existing at this point, the majority of graphics are poorly drawn and mis-colored.
Hack of Tom and Jerry on NES that replaces some of Jerry's graphics with Eevee.
Ver. 1: Hack of Tiny Toon Adventures where some of Buster Bunny's graphics are replaced with Pikachu, and the turning sprite after running fast is a garbled Mario sprite.
Ver. 2: Hack of Felix the Cat where Felix is replaced with a gray Pikachu.