Porthos (mirror)

Porthos of the Mirror Universe was the fierce, Terran canine domesticus, owned by Commander Archer. Aboard the Imperial NX Enterprise, he kept the Rottweiler mix-breed in his quarters; before commandeering the USS Defiant. Like his owner, he shared a common dislike and distrust of Captain Forrest, especially towards T'Pol. Up until recently, he wasn't allowed near the captain's ready room in this reality. (ENT episodes: "In a Mirror, Darkly")

Background Information
This fearless version of the regular universe beagle is a nod to Commander Kruge's targ, from "Star Trek III: The Search for Spock". Also in mannerisms; it would growl when needing to be fed and was only aggressive with the crew, not the master.