The Hirogen are a warrior race in the Delta Quadrant with more than a passing nod to Predators. The Hirogen like to hunt their prey and what they do with it afterwards is unknown. The Hirogen wear masks for most of the time. The first time the crew of Voyager come into contact with the Hirogen is when a patrol is tracking a Species 8472.
The Hirogen return later having taken control of the USS Voyager and have forced the crew to act out a World War 2 scenario with the Voyager crew as resistance fighters. Once the crew have broken out of the role play, Captain Kathryn Janeway gives the Hirgoen technology to create virtual worlds of their own.
Voyager having left the Hirogen area come into contact once again with the Hirogen when they find one of the Hirogens battle plays has got out of control.