Thanks to A-level biology and Chambers English Dictionary, I can suggest the following: Newt, narwhal, nagapie (bushbaby), naga (snake), nerka (sock-eye salmon), nandine (West African palm-civet) natterjack toad, nautilus, nematode (worm), nit (headlouse), noctule (great bat), noddy (sea bird), nuthatch, nightjar, nutria (coypu), nyala (antelope) - that should be enough to start you off!
I wasn't sure if you meant just mammal-type animals, if you see what I mean - anyway 3 more : Nilgai (Indian Antelope), Noolbenger (Honey Possum) and Numbat (Banded Ant-Eater).