My big kids both pretend to be dogs. They put pretend leashes on each other and all. DS1 is most interested in it, and will lick us like a dog, he will also eat food off a plate off the floor and bark, ect...I don't worry, it's just a 'thing'. Just pretend play, like pretending to be princesses or pirates. If it goes too far, just stopp it and tell him why he shouldn't do what ever it is. But to me it's all harmless fun.