Confusing and Often Misused Plurals
The plural of octopus is not octopi! "But," you say, "it ends in -us, and the plural of amicus is amici." Ah yes, but octopus isn't a Latin word; it's Greek! The -pus ending is short for pous, the Greek word for foot. If it were Latin, it would be octoped. Compare Oedipus, "swollen foot." So the most Greek plural of octopus is octopodes (four syllables), or, if you prefer, octopuses. The other Greek root for foot is pod, hence arthropod ("joint foot"), and cephalopod ("head foot"). The Latin root is ped, hence quadruped ("four foot"), and pinniped ("feather foot," referring to seals and their relatives).
Similarly, the plural of iris is irides (three syllables). Well, you could also say irises, if you must.
What's a kudo? There's no such thing. The word kudos, which looks plural to speakers of English, is actually singular. It's the Greek word for praise. The Greek plural would be kudoi, but we don't use it in English.
Remember the "neuter rule" in Latin, that all neuter nouns have plurals ending in -a? The same applies to Greek, so phenomenon is singular, and its plural is phenomena. The same is true of criterion and criteria.
Here's another one: What is the plural of ignoramus? Is it ignorami? Nope. In Latin, it's not even a noun; it's a verb meaning "We do not know." So, since it's only a noun in English, the English plural is ignoramuses.