A Delta Quadrant species who used holograms called isomorphic projections for maintenance work aboard their ships, one of which, Dejaren, killed his entire six-person crew before Voyager answered his distress call in 2374.
The Serosians are a spacefaring people of the Delta Quadrant. Their vessels carry isomorphic projections, holograms built to do dangerous and menial jobs such as cleaning reactor cores, and the crews treat them as equipment.
In the episode Revulsion, the Doctor and B'Elanna Torres answer a call for help from a Serosian ship and find only the hologram Dejaren aboard. He claims the crew died of a virus, but Torres discovers the bodies below decks: Dejaren, confined for years to a few compartments and full of loathing for organic life, murdered all six of them.
No living Serosian appears on screen. Dejaren attacks Torres when she tries to shut him down, and she and the Doctor destroy his holographic matrix before returning to Voyager.