Mammals:
* Narwhal
* Newt
* Nile crocodile
* Nightingale
* Norwegian elkhound
* Numbat
* Nene goose (though technically a bird)
Birds:
* Nightingale
* Nene goose
* Nightjar
* Nuthatch
Fish:
* Neon tetra
* Needlefish
* Nurse shark
Insects:
* Nymph (various insects)
Other:
* Nudibranch (sea slug)
* Nematode (roundworm)
* Nautilus (cephalopod)
This is not an exhaustive list, but it gives you a good starting point!