My earlier post is not particularly coherent, so here is the gist of it:
English has a lot of crazy plurals, especially for words ending in -us, because they started out using the plurals of the languages from which they were taken and have slowly shifted to using English pluralization rules. So you have nucleus -> nuclei (Latin second declension), lusus -> lusus* (Latin fourth declension), and octopus -> octopodes (Greek), but then bus -> buses (already plural in Latin, so it uses the English rule), circus -> circuses, and campus -> campuses (originally Latin, but assimilated). If you’re using substantives (adjectives acting as nouns) those don’t pluralize, so you have things like righteous -> righteous. It’s a mess.