From Avalanche: “Whether youve beaten the game or are just starting out new, Rivals will add a whole new type of challenge for all of you out there. As you fight your way through Östertörn the Machines will start taking extra notice as you destroy more and more of them. Destroy enough and one may evolve to become even more dangerous than usual. These Regional Rivals will be designated by a name and a title that hints at the origin of their evolution. Rivals can additionally be created should they defeat you – these Personal Rivals will have evolved due to their prowess in combat against the humans. Like the Regional Rivals, Personal Rivals will also be designated by a name and title that corresponds to their exploits.”
Source: N4G PC Generation Zero October "Rivals" Update Now Live