Porthos (mirror)

From Memory Alpha, the free Star Trek reference

Porthos, a Rottweiler, was Commander Jonathan Archer's pet dog in the mirror universe. Unlike his counterpart in our universe, this Porthos was not a Beagle.

Porthos was presumed killed in 2155 when the ISS Enterprise was trapped and destroyed in a Tholian web. (ENT: "In a Mirror, Darkly")

[edit] Background

This is the first instance of non-identicality of lifeforms with their analogs, although absences, such as no analogue 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.

In the 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."