Star Trek: 10 Things You Didn’t Know About Dax

Aside from the nine primary hosts, the Dax symbiont was also briefly transferred to the Trill Verad, in the episode Invasive Procedures. Verad dedicated his life to the goal of becoming joined with a symbiont. He was absolutely devastated when he was denied for symbiosis. Trying to escape a life of mediocrity, Verad and some accomplices invaded the station and forced Bashir to transfer Dax from Jadzia's body to his own. Eventually, the crew was able to return the symbiont to Jadzia, but she was left with permanent memories from Verad.

Strangely, the Verad personality didn't resurface during Dax's Zhian'tara in Facets, so it's possible their brief joining didn't affect the symbiont as profoundly as the other, more permanent hosts.

Additionally, we saw another Dax host named Yedrin in the alternate future from Children Of Time. This host was, however, removed from history when the crew of the Defiant prevented the ship from crashing on the planet Gaia, causing the timeline to change.