Random Thought-Plural "us"

So, we know that most words, when pluralized, will end in usualy an "s" ir "es". However there are exceptions. Matrices is the plural of matrix. Sure it still ends in "es" but we have to swap out the "x" for a "c". All in all, thats not too much of a stretch. Then there are the words that end in "us" like octopus or radius. We would like to say octopuses or radiuses but that would be incorrect. The correct plurals would be octopi or radii. This does not come to mind very quickly when asked this question. Oh well, that would mean the plural of Toyota's hybrid would be Prii and not Priuses.