"The One" was an ancient spaceborne lifeform, non-sentient, and the size of a class M planet. It shared a symbiotic relationship with another smaller, insect-like species called "the Many". Both species were believed to have existed in an earlier universe, prior to the Big Bang, given the unusual nature of their subatomic structures.
The One operated solely on instinct, compelling it simply to feed. To this end, the Many would seed target worlds with a certain strain of purple waterborne algae, and create planetary conditions for the algae to grow. To promote these conditions, the Many, through billions of years of evolution, developed an ability to manipulate advanced technology controlling nuclear weapons, by which they could provoke the release of large amounts of atmospheric radiation. In turn, the One would carry large colonies of the Many across interstellar space and closer to sources of the hard radiation they needed for survival.
The One was discovered by the USS Enterprise following the Talin IV disaster, en route to the Talin system, approximately half a light-year distant, and traveling at a velocity which would bring it to the system circa 2329. Although Lieutenant Chekov and Doctor McCoy were in favour of destroying it to prevent it from consuming the planet, Captain Kirk recognised that the creature couldn't be punished as it was no more evil than a shark who couldn't tell the difference between a fish and a swimmer. Having discovered the nature of the being, it was determined that the Starfleet Corps of Engineers would use the time before its arrival in the Talin system to convert the gas giant Talin VIII into a food source for the One, in order to protect Talin IV. (TOS novel: Prime Directive)