Kimberly H. reports that the plural form of octopus has taken a beating at the hands of the Roman invaders. Then there's this, from the Oxford English Dictionary site:
SPOGG firmly believes one can be both funny AND absurdly pedantic. So there!
"English words of Latin or Greek origin have rather unpredictable plurals, and each one usually depends on how well established that particular word is. It may also depend on whether the Latin or Greek form of the plural is either easily recognizable or pleasant to the speaker of English.
Although it is often supposed that octopi is the 'correct' plural of octopus, and it has been in use for longer than the usual Anglicized plural octopuses, it in fact originates as an error. Octopus is not a simple Latin word of the second declension, but a Latinized form of the Greek word oktopous, and its 'correct' plural would logically be octopodes.
Other words ending in -us show a very varied pattern. Like octopi, the plural hippopotami is now generally taken to be either funny or absurdly pedantic, and the usual plural is hippopotamuses. Common usage appears to indicate a slight preference for termini rather than terminuses, but syllabuses rather than syllabi. Other usual forms include cacti and gladioli, and our files at the dictionary department show scarcely any examples of nucleuses or funguses. (Omnibi is simply a joke, and quite ungrammatical in Latin!)
Among words ending in -um it seems worth drawing attention to the word curricula, plural of curriculum, and warning against confusion with the adjective curricular (as in extra-curricular)."