PLATYPI AND OTHER THINGS

When you are trying to figure out the plural of an English word ending in -us, consider its Latin declension.
Thus, we may classify our -us words into those of the second and fourth declension:

• Declension II: The most common one. Plurals end in -i: CACTUS/CACTI, OCTOPUS/OCTOPI, PLATYPUS/PLATYPI, HIPPOPOTAMUS/HIPPOPOTAMI (though HIPPO/HIPPOS has become more common...).
• Declension IV: This one, whose plurals end in -uses, is less common but nevertheless relevant: VIRUS/VIRUSES, LAPSUS/LAPSUSES.