Okay, so…
Continuing off sirpudding's mention of fake latinate words below, here's a quick lesson in plurals.
The standard English method of plural making is adding -s to the end of a word. If the singular noun ends in s, add -es. Simple enough.
There are irregular plurals, of course. The Latin plural is one of those. In Latin, a singular masculine noun ends in -us, and its plural ends in -i. (The singular feminine noun ends in -a and has a plural of -ae.) So, many Latin derived words follow that route instead: alumnus => alumni, radius => radii, and so on. Still, simple enough.
However, not all words ending in -us are Latin words. "Octopus" is the most obvious example, which actually comes from Greek with a Latin wash. But there's a whole other class of words in the English language that end in -us that aren't Latin derived: fake Latin.
Words like "cactus" and "nexus" weren't in Latin, originally, but were coined by 17th century scholars, using Latin (or in the case of cactus, Latinized) words as the basis to make them sound more erudite. As a result, it's sometimes okay to go cactus => cacti, but it's also okay to go cactus => cactuses. You don't generally see nexus => nexi, though, but nexus => nexuses.
ETA: Of course, sirpudding goes into more specific Latin detail, so listen to him.