- mollusc
- moonfish
- moorfowl
- moose
- mouse
- multiparous
- murex
- musk-cat
- musk-rat
- murices
- musk-deer
- mussel
- mustang
- myriapod
- narce
- narwhal
- musk-ox
- musquash
- naja
- nandu
- natterjack
- nautili
- nelly
- nervelessness
- nautilus
- nematode
- nerveless
- neuropterous
- neuter
- neutral
- newt
- nick
- nightingale
- nightjar
- noctuid
- nictating membrance
- nilgai
- noctule
- noddy
- nose
- nucivorous
- nose-ape
- nose-monkey
- nude
- numb-fish
- nurse
- nutcracker
- nylghau
- nun
- nutria
- nymph
- ocellate
- ocelli
- octopus
- oculated
- odd-toed
- oculat
- odonate
- okapi
- oligopod
- omnivorous
- onager
- onagri
- onchosphere
- oncosphere
- onychophoran
- opah
- opisthbranch
- opisthoglypha
- opisthognathous
- opossum
- obtect
- ocellated
- ocellus
- oligochaete
- omnivorousness
- orang
- orang-outang
- orang-utan
- ornithorhynchus
- orohippus
- oriole
- ormer
- orthoptera
- orthopteron
- orthopterous
- ortolan
- ostracea
- ostrich
- ounce
- ousel
- ouzel
- oryx
- osprey
- ostracion
- ostracode
- ostracoderm
- otter
- ouananiche
- outbreed