A 22nd-century beagle, Porthos is Captain Archer's closest companion and confidant. The four-legged space traveler lives aboard the Enterprise NX-01, remaining primarily in Archer's quarters and the Captain's Ready Room. But he does occasionally get to join an away mission, and if there's an alien tree nearby, he will boldly go "where no dog has gone before."
Archer takes full responsibility for Porthos' care and feeding, and occasionally indulges him with treats of cheese, which he can't get enough of. Unfortunately, dairy products have an adverse effect on Porthos, prompting the occasional visit to Dr. Phlox for gastrointestinal distress.
Porthos is friendly with every member of the crew, even the aloof T'Pol, but he is also an effective watchdog when unwelcome guests invade the captain's quarters. On one such occasion, Porthos was nearly kidnapped by some strange, large-lobed aliens, who couldn't decide if Porthos was "the captain's next meal" or some sort of higher lifeform.
Porthos also accompanied Archer on the captain's first vacation from Enterprise. While relaxing on the famed pleasure planet of Risa, Porthos had a run-in with another canine. Even when the captain faces dangerous situations, he keeps Porthos in mind — when Archer was forced to surrender to Silik, he asked Ensign Hoshi Sato to look after the pup in his absence.