All the English nouns with invariant plurals denote animals, deer, sheep, fish, swine, which are eit
All the English nouns with invariant plurals denote animals, deer, sheep, fish, swine, which are either herded or hunted; and the ‘mass noun’ sense with herd animals as well as the custom of referring to all hunted animals in the singular helped inhibit plural regularization in the transition from Old English.And people said that English degree would never do me any good. -- source link