The Shih Poo dog is a hybrid that resulted from intermixing the Shih Tzu and Toy Poodle. Although this breed is already popular, the kennel clubs still don't recognize it as an official breed. Because of this, the Shih Poo has no breed standard and so every appearance and disposition is accepted by the community.
Dogs are surprisingly complex creatures. Estimates about the number of breeds are as high as 800 in Western countries alone. Even given the fact that the difference from one breed to another can be carried to absurd extremes, the variety is astonishing from a human perspective, who have, perhaps, only a dozen 'breeds'.