According to Marlena Moreau, "I always thought that was funny, the great, powerful CaptainKirk who owes everything to some unknown alien scientist and a plundered laboratory." James T. Kirk justified his (counterpart's) actions, by beginning to state that "Well, if you don't take advantage of your opportunities…" before Moreau finished, "…you don't rise to the command of a starship or even higher." Kirk later used it in his rise to power by killing his many enemies. (TOS: "Mirror, Mirror")
When that ship was stolen from a Tholian dock and commandeered by Jonathan Archer, an unknown reptilian creature attacked and killed Kelby, one of Archer's officers. The alien slave worker was interrogated in the Defiant's briefing room as to the identity of the murderer. Eventually, he revealed that the killer was his slave master, Slar. (ENT: "In a Mirror, Darkly, Part II")
The alien and his species were unnamed in both the episode's script and the episode itself, where he was played by Pat Healy. His immediate fate after being interrogated is unknown, but assuming he wasn't killed by Archer he would have been confined to the ISS Avenger with the other non-Terrans, and then killed when that ship was destroyed.
He appears to have the same makeup, albeit with a different coloration, as several Rigel X aliens.
When the ISS Enterprise crew was attempting to commandeer the Tholian-held USS Defiant, they destroyed a Tholian vessel and captured the pilot, beaming it into the ship's decon chamber. Phlox proceeded to torture the Tholian by lowering the temperature in the chamber until it gave them the location of the Defiant. When the Tholian later attempted to send a signal to its people, Phlox killed it by lowering the temperature until its carapace shattered. (ENT: "In a Mirror, Darkly")
This CGI Tholian was the first Tholian's full body form ever shown. The movement actor who "performed" the role during filming and provided the basic movements for this character was played by extra David Keith Anderson, who received no credit for his work.