I think 'sheep' is the only animal noun that can't take the -s morpheme for plural, but I won't swear to that.
Herein NZ, there is a battle of sorts going on over pluralising the names of native fauna. Maori does not add "s", so in Maori, one would say "rua kiwi" for "two kiwis". The argument is related to the debate over PCness, with some saying that the words in question are now part of NZ English, and so shoud be pluralised in the standard English fashion, while others say that Maaori conventions should be followed. The word that is most consistently not pluralised is Maori itself, it is now common to hear one Maori, two Maori.
p.s. The double "a" in one instance of "Maori" above was initially a typo, but I left it in because that is how it should be spelled when using fonts that do not include vowels with macrons above them, used in Maaori to indicate long vowels. I do not normally follow the doubling rule because it looks ugly.
