We know the mortals were all once Ehlnofey, and supposedly all monsters can trace their roots back to Bosmeri Wild Hunts, but what in Oblivion were the animals? Do they all come from Kyne? Were they mortal souls filtered into beastly bodies by the Dreamsleeve? What happens to animal souls when they die?

Maybe Magnus or Lorkhan believed that in order for a planet to function in a capacity that would allow mortal sentient species to exist (a persevere through adversity), it would require a biosphere made up of a variety of flora and fauna, constructing a food chain and self-sustaining balance.

They could have been in the blueprints from the start or they could have been an unforeseen development from the original design that arose by coincidence like life evolved by natural selection on our planet.

It's impossible to say for sure without one of the writers giving a statement (if they haven't somewhere already).