(coexisting on another dimensional plane)
In January 2155, Porthos was present in Archer's quarters, growling at Sergeant Travis Mayweather, 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.
Background information Edit
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 Vic Fontaine 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."