There are many words of Latin or Greek origin which end in ‘-us’, however it is not always clear what the plural form is. Do words like ‘hippopotamus’ and ‘octopus’ end in -es or -i? The website About.com explains that it is best to go with hippopotamuses http://homeworktips.about.com/od/plurals/f/hippopotamus.htm, whereas with the word octopus you apparently cannot go wrong if you guess the plural as octopuses, octopodes or octopi http://homeworktips.about.com/od/plurals/f/octopus.htm as they suggest that all of them are correct.
I found the best coverage about this subject on Wiki answers: http://wiki.answers.com/Q/How_do_you_make_words_ending_in_us_into_plural. They explain that you add -es to most words that end in -us; however, “words that come from latin, such as stimulus, nucleus, and radius, still act as they did in latin. That is, they end in ‘-i’: stimuli, nuclei, radii”. There are exceptions like virus where the plural is viruses – the reason for this is explained in the Wiki answers link above.
My thoughts: This was not as straightforward as I thought, although I have learnt a few new rules about awkward plurals today.
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Until tomorrow…
Sandra
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Reference list:
About.com: http://homeworktips.about.com/od/plurals/f/hippopotamus.htm
About.com: http://homeworktips.about.com/od/plurals/f/octopus.htm
Wiki Answers: http://wiki.answers.com/Q/How_do_you_make_words_ending_in_us_into_plural
My Writing Notebook 
That’s why there are no hippopotamuses or octopuses in my novel. To darn hard to pluralize. Isn’t the English language annoying sometimes? Even when you take the time to do it right, it sounds wrong.
Posted by Jennifer M Eaton | September 26, 2011, 3:36 am