The rule is simple enough. Words of Greek origin are made plural with -es and words of Latin origin are made plural with -i. For example, octopuses, cacti. But is ´syllabus´ Greek or Latin?!
And so while I´m driving I look it up in the Online Etymology Dictonary. I found the story so funny, I´ve decided to share it here. The confusion is all based on a mistake!
syllabus (n.)1650s, "table of contents of a series of lectures, etc.," from Late Latin syllabus "list," ultimately a misreading of Greek sittybos "parchment label, table of contents," of unknown origin. The misprint appeared in a 15c. edition of Cicero's "Ad Atticum" (see OED). Had it been a real word, the proper plural would be syllabi.
But it wasn´t a real word, so the proper plural* is, in fact, syllabuses!
*Well, that depends on if you´re a descriptivist or prescriptivist. Language is evolving and in time, the common vernacular becomes what is correct blah blah blah.