The Nemesis is a very large zombie-like creature originally portrayed in the Resident Evil series of video games. He is also a false myth in Grand Theft Auto: San Andreas and is referenced in Grand Theft Auto V.
Grand Theft Auto: San Andreas
Players believe that the Nemesis can be found in GTA San Andreas. He is thought to appear inside the Zombotech Corporation Building, standing inside one of the elevators. Players reported that Nemesis attacks them with a rocket launcher if they get close to him, and he will continue to pursue the player until they die, it is believed that the Nemesis has a lot of health and can't be killed easily. Players also have modded the creature and showcased it to Youtube, and is available for download on various modification websites. Furthermore, there is very little evidence to back up the myth, making the existence of the creature false.
Grand Theft Auto V
In Grand Theft Auto V, there is a motorcycle named Nemesis. That could be, however not confirmed to be a reference to Nemesis.