Porthos was Captain Jonathan Archer's pet beagle. One of four puppies in a litter, Porthos and his brothers were named after the main four musketeers from the Alexandre Dumas novel The Three Musketeers.
Porthos lived with Archer on Earth in mid-2150. In the same year, Archer was selected to command the NX-class starship Enterprise.
When Archer moved aboard Enterprise in April of 2151, he took Porthos with him.