Zeltrax is actually Terrence "Smitty" Smith, an old associate of Tommy Oliver's who holds him responsible for the recent tragic events in his life, including his near fatal accident. Mesogog found his ruined body and rebuilt him with cybernetic technology into a black armored cyborg. As Zeltrax, he is bent only on getting revenge on Tommy, much like Tommy's relationship with Goldar several years before. During his attempts to accomplish this, he inadvertently created a clone "son" named Goldenrod. Goldenrod proved a powerful adversary to the Rangers, but was destroyed by Tommy, increasing Zeltrax's hatred of the Black Ranger.
After a second near fatal experience because of his master's plans, Zeltrax turned against Mesogog, fleeing his side and gaining a super powered form through the special underground spring that flowed beneath the Tree of Life. Using the Triptoids, Zeltrax built his giant Zelzord, which was destroyed by the self-destruction of the Rangers' zords. In the finale Zeltrax met his end fighting Tommy and Kira, who destroyed him despite his attempt to destroy the Rangers.
In Power Rangers S.P.D., Zeltrax reluctantly joined forces with Emperor Gruumm who had traveled back in time through a temporal wormhole, presumably to a point in 2004 before the events of Dino Thunder's season finale. Together they fought the Dino Thunder Power Rangers and the S.P.D. B-Squad Rangers. Despite Gruumm's aid, Zeltrax was defeated by a double-team from Tommy and Anubis Cruger, the SPD Shadow Ranger.
Zeltrax was voiced by James Gaylyn while the actor for Smitty was uncredited. Coincidentally, Zeltrax's cloned "son" Golden Rod is also voiced by Gaylyn.
Read more about this topic: Villains In Power Rangers Dino Thunder