Don't let any arrogant cactuses tell you that "it's not cactuses, it's cacti"...
It would be "cacti" in Latin, but we're not speaking Latin. It's a Latin-rooted word being used as a loan word in English, so it becomes subject to the pluralization rules of English, which means "cactuses" is perfectly correct. Same with "octopuses", "statuses", "funguses", and yes, even "alumnuses". Just the same, there's a lot of nutty purists out there that think if we want to pluralize Japanese loan words like "ninjas", "samurais", or "katanas", we can't put an s at the end because Japanese doesn't have plural forms. But we can!
