That or they all four walk on four legs.
4 legged answer is too simple... they are all also hoofed except kangaroo, and mammals where kangaroo is marsupial, all these facts together combine to make them more similar to each other than to 'roo. also, all are domestic (they have deer farms and wild horses but you know what i mean), whereas roo is still undomesticated.
nothing wrong w/the simple answer, dru. simple minds, etc...
i just didn't want to unscramble the word....laziness prevails!