ehh, yes, that's what I meant with the sentence: "Some words are just inconsistant..."
I search google and came up with:
"In English, plurals of nouns are normally indicated by the ending –s or –es, or in a few cases by –en, as in children and oxen."
and
"•The absence of –s in the plural form of animal names (hunting for bear, a herd of buffalo) probably arose by analogy with animals like deer and sheep whose plurals have been unmarked since the earliest beginnings of the English language."
But there always are exceptions. two dogs/cats/horses.
I don't think either that there is a "end it all"-rule