English has no trouble coöpting or adapting words from other languages. If that word is a noun, we usually just drop an "s" or "es" on the end for plurals—regardless of how pluralization is done in the original language. But for some reason, this rule just does not follow with many Latin words. Vertebra becomes vertebrae; medium becomes media; curriculum becomes curricula; cactus becomes cacti. And yet bonus becomes bonuses and album becomes albums. Some are interchangeable; syllabus can be either syllabuses or syllabi. The more pedantic the speaker, the more likely they will use the later.
It is capricious which words are usually pluralized according to English rules and which are pluralized using Latin words.
Anglicized words should use standard English pluralization. Why the exception for Latin? Some of these words are probably too late to Anglicize, but I am going to continue to refer to syllabuses and cactuses. But if you hear me refer to "the mainstream mediums," I am (probably) being ironic.