In the Star Trek universe, the Hirogen are a large, violent race from the Delta Quadrant, first encountered by the USS Voyager. They like to hunt other creatures simply for the fun of it and they decorate their ships with "trophies" from the hunt. (Body parts, I believe.)
They have no home planet any more, having been flying around hunting for millennia. In the 2370s, Voyager provided them holotechnology to allow them to perform simulated hunts rather than actually killing sentient beings. This was hoped to give the Hirogen the ability to settle down in one region of space and rebuild their culture in a unified manner. Problems arose when the holographic prey proved too well programmed and began hunting the Hirogen, however the combiend forced of Voyager and the Hirogen managed to defeat the holograms.
Episodes
The Hirogen appear in nine episodes of Star Trek: Voyager during seasons four through six): Message in a Bottle, Hunters, Prey, The Killing Game, The Killing Game, Part II, Infinite Regress, Tsunkatse, Unimatrix Zero, Part II, Flesh and Blood.
They are mentioned in five other Voyager episodes: Message in a Bottle, Hunters, Prey, The Killing Game, Part I, The Killing Game, Part II, Infinite Regress, Tsunkatse, Unimatrix Zero, Part II, Flesh and Blood.
Sources:
http://memory-alpha.org/en/wiki/Hirogen
http://en.wikipedia.org/wiki/Hirogen