Lifeforms made of photonic energy, met by Voyager both inside a protostar in Heroes and Demons and as visitors from another dimension in Bride of Chaotica!
Photonic lifeforms are living beings that consist of light rather than matter. Voyager meets two unrelated kinds.
In Heroes and Demons, set in 2371, the ship takes samples of photonic energy from a protostar and unknowingly captures a living creature. Others of its kind enter the holodeck, where Harry Kim is playing Beowulf, and convert Kim, Tuvok and Chakotay to energy as hostages. The Doctor, himself a creature of light, enters the programme and returns the captive, and the crewmen are released.
In Bride of Chaotica!, set in 2375, explorers from a photonic dimension come through a rift into Tom Paris's Captain Proton serial. They cannot detect carbon-based life and believe the holodeck characters are real, so they go to war with Dr Chaotica. The crew has to play out the story to its end to free the ship.