It is a word borrowed from Spanish, according to merriam-webster. Therefore, the plural will be nutrias according to the rules of Spanish. All those other words, though probably latin or Greek in origin, have been in the English language long enough that they are no longer borrowed words and therefore follow English grammar rules.
Did you read a dictionary?
https://www.merriam-webster.com/dictionary/nutria
It says there, as well as in the Encyclopedia Britannica someone linked earlier, that “nutria” is the singular form.
That’s clearly a mistake on their part that requires correction.
It is a word borrowed from Spanish, according to merriam-webster. Therefore, the plural will be nutrias according to the rules of Spanish. All those other words, though probably latin or Greek in origin, have been in the English language long enough that they are no longer borrowed words and therefore follow English grammar rules.
No, “bacteriums” is not a word.