The El-Aurians were a humanoid civilization of "listeners" with extremely long lifespans. Most of them were assimilated by the Borg in the year 2265, the few survivors being scattered throughout the galaxy.
El-Aurians had the notable ability to sense changes in the time stream resulting from time travel, as well as disturbance events that occurred outside normal time. This was similar to the abilities of a Time Lord. (TNG episodes: "Yesterday's Enterprise", "Time's Arrow, Part I", "Time's Arrow, Part II"; TNG - Assimilation² comic: "Issue 4")
In 2293, a group of El-Aurian refugees were being transported by Federation ships when they were caught in the Nexus. They were reluctantly rescued by the USS Enterprise-B, although at the apparent cost of the life of James T. Kirk. (TNG episodes: "Q Who?", "Time's Arrow, Part I", "Time's Arrow, Part II"; DS9 episode: "Rivals"; ST movie, novelization & comic adaptation: Generations)
In 2379, while on board the USS Enterprise-E, Guinan revealed to Captain Jean-Luc Picard that the El-Aurians were actually the first species to be assimilated by the Borg. As a result of Vulcan assimilation, the Collective uncovered a logical paradox in their philosophy of perfection: no perfect species could exist from one which was created imperfect. Thereafter, the Borg initiated a series of changes in the past by attempting to assimilate lifeforms from the time of inception. By assimilating the El-Aurians first, the Borg also assimilated the capability to remember each subsequent shift in time—so as to further isolate the exact point where life began. (TNG - Alien Spotlight comic: "Borg")