Could it be that these are animals that were hunted, fished, or herded and that there was a convention that used the singular as a sort of generic plural when going after these creatures for sport or ...

Do you know how we can turn a singular noun into a plural noun? The solution is simple. Just add an '-s' at the end of the word, and whoosh, the job is done. The word 'house' becomes 'houses', 'word' ...