Porthos (mirror)

Porthos was a dog belonging to Commander Jonathan Archer, the first officer of ISS Enterprise (NX-01) between 2151 and 2155. 

Before In a Mirror, Darkly
Porthos was known amongst the crew. In January 2153, Commander Tucker used the fact Porthos had a name yet a Vissian cogenitor did not to try to convince Commander T'Pol to help him liberate the cogenitor. (Before In a Mirror, Darkly: Cogenitor)

After Enterprise returned to Earth following a surprise Xindi attack on the planet, Phlox intended to use Porthos to protect himself whilst taking a walk around San Francisco, convinced even the worst xenophobes would not attack Archer's dog. The theory was never tested as the crew returned to the ship immediately. Later, Archer and Porthos stood by when Veylo, a Vissian held prisoner on the ship, escaped and held the catering staff, including Chef, hostage in the mess hall. Following Tucker's shooting of Veylo, Porthos killed her, which saved her from further pain at the hands of the Enterprise crew. (Before In a Mirror, Darkly: A Crisis on Earth)