Pluralization of Latin-based nouns is a complicated field. Preference for Latin or English plural endings is inconsistent in similarly constructed words, as is the presence of alternative forms at all. Here’s a guide to plural forms for Latin words, identifying, for more than a hundred nouns ...
"Echo" becomes "echoes" or "echos." The plural of "volcano" can be either "volcanoes" or "volcanos." Unfortunately, there is no clever way of knowing which nouns ending "o" follow which rules. You have to know. (For example, you have to know that "solo" becomes "solos," but "...