Star Jelly

Star Jellies are sapient, starship-sized creatures which live and travel in interplanetary space and are able to directly convert energy into highly elaborate patterns of matter, much like a replicator or transporter. Their natural form is that of a luminous saucer-shaped organism with a cluster of tentacles projecting from the disk's center. They are known to travel in pairs and appear to touch tentacles to communicate, or simply to express affection. Their matter-generating abilities allow them to shapeshift into a fully functional starship or station.

Behind the scenes
These creatures appeared unnamed in the pilot episode of Star Trek: The Next Generation and were never properly named in canon. The designation "Star Jellies" comes from the Star Trek: Titan novels where they are also referred to as a type of cosmozoans (a generic term including several distinct species such as the Crystalline Entity, the Space Amoeba and Junior's species). They belong to an entire ecosystem of cosmozoan. They are hunted by an alien species known as the Pa'haquel who butcher their bodies to create vessels for them to travel in space.