Athos was a beagle dog that lived in the 22nd century.
Named after a musketeer from the Alexandre Dumas novel The Three Musketeers, he was the brother of Jonathan Archer's Porthos. He also had two other brothers, named Aramis and D'Artagnan. Athos and his brothers were born to a female dog, owned by the mother of one of Archer's ex-girlfriends. (ENT: "A Night in Sickbay")