My dog would definitely have been a Gryffindor--outgoing, full of energy, eager to protect us, and always sure he could challenge any other dog, regardless of whether or not it was way bigger than he was. My cat, on the other hand--definitely Slytherin--cool, elegant, clever (he kind of looks like a Malfoy, if Malfoys were cats)--we used to live in a house with other cats and I would catch him tricking some of the other cats out of their food when no one was watching.