Nouns ending in - al change to - aux in the plural, eg: ...
English is full of irregular plural forms based on Latin and Greek. They can be confusing (apparatus? apparati? apparatuses?). They can be fun (the brothers Winklevii! and the flying Elvii! all ...
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 ...