Edited sentence: For Latin fifth-declension nouns, such as manus, the singular and plural forms both end in -us.For third declension nouns such as octopus, the plural is less regular.However, this is only correct for Latin masculine nouns of the second declension.Its plural in Latin is actually octopodes, not octopi.The noun octopus in Latin is a third-declension noun borrowed from the Greek.The noun octopus is often made plural in English as octopi, originally from the mistaken belief that all Latin nouns ending in -us take -i to form their plural