I think 'sheep' is the only animal noun that can't take the -s morpheme for plural, but I won't swear to that.

and some plurals refer (in my mind anyway) to different types of animal, so "fish" can be used as a general plural but "fishes" refers to some cod, some bream, etc. I have heard this applied to sheep as well, but it sounds awkward so is probably wrong (Somone's First Law of Language?).



Rod Ward