At the age of six weeks (a little early IMO) Captain Johnathan Archer had his trusty English Beagle, Porthos. In Season 1 Porthos was played by a male beagle named Prada, whom also played a beagle named Lou in the film Cats and Dogs (voiced by Toby Maguire). In later seasons two female beagles shared the honor, Breezy and Windy (although Porthos was always a male).
In one of the final episodes of Enterprise, “In a Mirror Darkly” (a “evil twin universe episode”), Porthos was replaced with a Rottweiler. While they tried to find a dog that would be a polar opposite to the Beagle (and Rotties do have that look), on set reports that the Rottweiler was even a bigger baby than the Beagle – writer Mike Sussman noted that he was “The sweetest dog I Have ever met”.
In the new Star Trek Film (2009) there is reference to Montgomery Scott having a “transporter accident” with Admiral Archer’s dog. I highly doubt it was the beloved beagle since he would have been over 70 years old when Scotty was born. 🙂