Nobody said they have to be exact copies of real world animals, but they should represent the most prominent aspects of an animal, and with a kangaroo (imho) it's speed, jumping and kicking, not that it carries its baby in a pouch, most animals around here do that. Kangaskahn is more a general marsupial pokemon than a kangaroo.
And: I hate it when people say "animal x is already represented by pokemon x we don't need it anymore". Why? Why should the existence of Kangaskahn for all eternity forbid the creation of a more agile interpretation of kangaroos?
With enough creativity one animal can lead to a number of completely different designs, each representing a different, extraordinary aspect of the original animal and mixed with other concepts. Most types of real life animals have many different species and/or breeds, why should pokemon be restricted to "one pokemon per animal'?
If we can have a redundant Pikachu clone every generation (seriously Dedenne?) and three butterflies, then we can have two kangaroos.