Porthos (mirror)
Porthos was a Rottweiler and a pet of Commander  in the mid-2150s of the 22nd century, and resided within Archer's quarters aboard the  flagship .

In January 2155, Porthos was present in Archer's quarters, growling at Sergeant , which resulted in the Terran glancing towards Porthos. His owner informed Mayweather that Porthos had not "had his dinner yet" and to therefore pay him no attention.

Archer later patted Porthos while discussing the factual truths of orders from  with . ()

Appendices

Background information 
The final fate of Porthos was not revealed as of . However, the absence and lack of mention in the second part of the two part episodes, and Commander Archer being off ship at the time, may mean that Porthos was killed when ISS Enterprise was destroyed inside the Tholian web. The  stated that Porthos was apparently killed with the destruction of Enterprise.

This is the first instance of non-identical lifeforms having the same name and role between the two universes, although absences, such as no analog of Jake Sisko, and physical differences – such as the fact that  was a solid being – have occurred from time to time. In this case, the difference in breeds is an indication of the differences (and similarities) between the personality of Jonathan Archer and his mirror. The mirror Archer has chosen a different breed of dog as a pet, but still decided to name him Porthos. Thus this dog is most likely not a mirror version of Prime Porthos, rather a different dog that was given the same name by mirror Archer. 

In the final draft script of "In a Mirror, Darkly", Porthos was described as "a large, imposing-looking CANINE (possibly a rottweiler or pit bull)."

In a podcast of the episode on StarTrek.com, it was said that the dog who played "the vicious Porthos" was actually a bigger baby than that of the original Porthos. He was noted by Mike Sussman as being "the sweetest dog I have ever met."

Apocrypha 
In the Star Trek Online mission "Vorgon Conclusions", the player raids a Starfleet research outpost containing a number of strange relics acquired throughout the years. One is an NX type escape pod found orbiting Luna in the 2370s, which apparently contained mirror Porthos. He was subsequently adopted by Reginald Barclay.

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Dogs
ISS Enterprise (NX-01) personnel