Ah, it's this issue again.
Not every word with the ending
-us is of Latin origin, and though most that
are of Latin origin are second declension nouns (for which
-i would be a correct plural ending), there are exceptions. Such as
domus, which is an irregular fourth declension noun.
But words such as
octopus and
platypus are actually derived from
Greek, not Latin, roots, and the declensional rules for Greek nouns are
much more complicated than for Latin. Though Greek has technically three, rather than five, noun declensions, Greek second declension nouns are divided into three subsystems, and third declension nouns into no less than
eight subsystems. That's before you discover the delights of irregular nouns, and the weird foibles that attach themselves to the use of various neuter nouns (which vary yet again when you move from Attic Greek to Doric, Ionic and Homeric poetic Greek).
From those Greek origins, the correct plurals (pedant hat on!) would be
octopodes and
platypodes (a consequence of the declension of the Greek word
πους, "foot"), and similar rules would apply to other Greek derived nouns according to the root words and their (frequently intricate) declensional details.
You can bet I'm a star at parties with this