Watson is a dog owned by the Foster (Freeman) Family. When Sophie chose to have her family's memories wiped, she requested that Alden give them a large yard so that they could get a dog. They ended up getting a beagle that her family named "Watson".
Watson, along with Marty, went to Atlantis with Sophie and Amy in Nightfall. He and Marty were quite vicious, shredding Alden's tunic sleeves to bits because he was bringing them to Atlantis and they didn't want to get wet.