Electromagnetic lifeform
According to Neelix, the electromagnetic lifeform discovered in a class J nebula in the Delta Quadrant was found to be an intelligent entity composed of electromagnetic energy. The , using its Bussard collectors to gather deuterium from the aforementioned nebula, accidentally caused the nebula to destabilize. The lifeform responded with hostility by boarding the ship and attempting to alter its atmosphere to better emulate its home environment, while simultaneously changing the ship's course in an attempt to return home. 

The electromagnetic lifeform's alterations caused power outages and forced the evacuation of many areas of the ship, including the bridge and sickbay. Several crew members were injured, some severely, by system malfunctions caused by the entity. Included among the injured were Crewmen Gibson, McMinn, and Mulchaey, as well as senior officers Ensign Tom Paris, Lieutenant Commander Tuvok, and Seven of Nine.

From a temporary command post in main engineering, Captain Janeway was eventually able to communicate with the creature, with it using the voice of the ship's computer to communicate. The lifeform requested to be returned to its home nebula and Janeway agreed. However, when Voyager returned to the nebula it was found to have dissipated completely. The lifeform was outraged and forced the evacuation of the entire ship by cutting off life support. Janeway tried to reason with the creature, offering to find it a new nebula, but it refused, intending to take the ship as its home. She informed it that the ship's technology, which it was using to maintain its needed environment, would soon fail without a crew to maintain it. The lifeform, however, insisted they evacuate and Janeway agreed to the demand. However, it prevented Janeway herself from leaving, intending to have her remain aboard to maintain the systems. But she flatly refused, even when it tried to force her by almost asphyxiating her. Seeing that it had no choice, the lifeform relented and agreed to let her find it a new home. 

An artificial environment mirroring its home environment was set up for it on Deck 12, Section 42 of Voyager. The area was made off limits to all crew except senior officers with security clearance level 6 and above. This started rumors of ghosts and monsters on that deck among the five children aboard the ship at the time. The story of it was told to the children by Neelix in 2376, when they drifted powerless in another nebula. ()

See also
Magnetic organism

Non-corporeals
Unnamed species
Lifeform classifications