My DS does this too, he is a dog, a cat, a frog, a dinosaur, a super hero (where I have to tuck one of our old cloth nappies down the back of his shirt so he can have a super hero cape) and just recently we bought him some guinea pigs, and he has pretended to be them too :-) I think it is gorgeous. I always have to be the mummy animal and he is the baby animal and I have to tell him what to do etc.. all in a days work for us mummies