The Borg existed in different continuities of the mirror universe.

Star Trek: New EmpireEdit

The Imperial Union of Planets sought for years to fashion mirror-equivalents of the Borg for servitude and soldiery. In 2385, Starfleet officers on the ISS Arcadia succeeded in creating Borg drones by obtaining Borg nanoprobe schematics from the Federation universe. These Borg soon ran amok and became a threat. Apparently their threat was neutralized by 2386, though in what manner remains unclear.

Star Trek: Unity (fan film series)Edit

The mirror universe Borg were a large group of cybernetically enhanced beings who helped their galaxy to overcome ecological problems. They were nowhere near as hostile as their primary universe counterparts.