Porthos

Porthos was a male beagle, the pet dog of Jonathan Archer. Archer took Porthos with him when he took command of the United Earth Starfleet starship Enterprise in 2151.

Porthos remained with Archer throughout Archer's entire tenure as Captain of the Enterprise, often going where no dog had gone before. (Star Trek: Enterprise)

History
He was one of four male puppies in his litter. He and his brothers, Aramis, and were named after the four central characters of the Alexandre Dumas novel The Three Musketeers. Rebecca's mother had raised the pups and gave Archer Porthos. 

Porthos was extremely fond of cheese and Archer often obliged him. However, ingesting cheese occasionally called him caused gastrointestinal distress. 

In 2151, he became the first inhabitant of Earth to set foot on a planet later named after his master: Archer IV. He immediately ran into the forest and proceeded to urinate. Commander Charles Tucker III quipped that he was really was going "where no dog has gone before." 

Later that year, Doctor Phlox provided Subcommander T'Pol with a numbing agent to protect her from the worst of Porthos' smell. However, she still found his smell offensive, particularly after Captain Archer had fed him cheese as it caused him to flatulate. 

While the crew of Enterprise were incapacitated by the Ferengi marauders Ulis, Krem, Muk, and Grish, Porthos remained conscious, presumably due to his non-humanoid physiology. Grish attempted to use his universal translator to communicate with Porthos but Muk soon chastised him for not realising that he was a "lower lifeform." Muk commented that he would likely fetch several bars of latinum at the Malurzian Zoo and placed him in a cargo container. After regaining control of Enterprise, Archer retrieved his pet, who appeared very happy to be reunited with his master. 

In 2152, Porthos was infected with a deadly pathogen while accompanying Archer, T'Pol and Ensign Hoshi Sato on an away mission to the Kreetassan homeworld. Phlox was forced to operate, replacing his pituitary gland with that of a Calrissian chameleon which he had in storage. He made a full recovery. Archer was later forced to carry out an elaborate ceremony of apology as, during the mission, Porthos had also urinated on a sacred tree. 

Porthos was aboard Enterprise for a full decade, remaining at his master's side throughout the Xindi Crisis, the Earth-Romulan War and until the ship was decommissioned in 2161. 

When Archer was promoted to admiral later that year, Porthos accompanied him to Starfleet Headquarters. He and Archer often visited Enterprise, which had become a museum piece in the Smithsonian Institution's orbital annex. By May 2163, Archer had begun to notice that Porthos had slowed down considerably due to his advanced age for a beagle.