Xen was a Trill symbiont, a symbiotic life-form that existed with humanoid Trill as a joined species. Each joining of Xen with a host created a new, unique individual, but each individual also carried the memories of the previous hosts.
According to Admiral Theseus, the Xen symbiont had hit legendary status as being the oldest Trill symbiont known to live. In 2375, its latest host was dying on the Breen homeworld. Commander Gotens temporarily joined with the symbiont in order to save its life, and for a short while, seemed to have contained a vast amount of wisdom because of it. The Xen symbiont was later brought back to the Trill homeworld, and Gotens was returned to his normal symbiont. (Star Trek: Phoenix-X: "Guinea Pigs")