In the mirror universe, the Trill are a humanoid species joined with symbionts.

In 2287, Spock ordered that all Trill joined with symbionts to be killed, although Curzon Dax's life was spared. In 2379, Curzon died, but not before Dax was transferred to Ezri Tigan. (ST - Mirror Universe novels: The Sorrows of Empire, Rise Like Lions)

Some Trill, such as Jadzia Idaris, were members of the Terran Rebellion. (DS9 episodes: "Through the Looking Glass", "Shattered Mirror")