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 Book 6: Nightfall. Apparently, he and Marty were quite vicious, shredding Alden's tunic sleeves to bits because they did not know him.
It can be noted that Watson is the name of Sherlock Holmes' companion, a common motive in the KotLC universe.