I've mentioned virus and octopus. Memiyawanzi noted that syllabus is derived from Greek σιττύβας (sittubas), accusative plural of σιττύβα (sittuba) "parchment label", so etymologically syllabus is already plural.
Another weird one is bus, which is a shortening of omnibus, which is the Latin dative plural of omnis "all". It is sometimes pluralized as omnibi, apparently humorously.
apparatus and status are borrowed from the Latin fourth declension masculine nouns apparātus and status, so the Latin plurals are apparātūs and statūs.
agenda, erotica, opera, data, media, bacteria, candelabra, paraphernalia, trivia, graffiti are all borrowed from Latin plurals (Italian in the case of graffiti), so English plurals like agendas and operas are unetymological, as is treating these words as singular nouns.
And so on. The point is of course that if we insist on pluralizing words according to how they are pluralized in their original language, we run into all kinds of problems - not the least of which is that we shouldn't have to know the morphology of another language in order to use our native language. I take the sensible view that if it's an English word, pluralize it like English - that is, with (e)s.