Ok, now follow me here... If you get lost, pull over, check the references, and begin again.
In the second declension of Latin nouns, you can see (in the "auxilium, –ī"), that a noun ending in "-um" in the singular form changes to "-a" in the plural form.
The name for a group of large rodents indigenous to South America is Nutria. Take this with the point above: Plural = Nutria, Singular = Nutrium
OMFG WHAT IS MOISTURIZING MY DOVE SOAP?>!?!?!?!11!1?!!?