English may not be the hardest language to learn but it certainly has its quirks. The rules for plurals are one of the areas where there are a lot of those quirks. There are more than a few places that will give you the the rules. On top of the rules you need to worry about the exceptions.
While the rules for plurals and the exceptions are important for using English properly. That's not the only reason I link to them. The real reason is because of cephalopods.
I really do like the video Oktapodi. If you haven't seen it give it a look. A charming story of the lengths one octopus will go to save its mate. Which leads to an important question: If you have more than one octopus what do you call them?
When in doubt... ask an editor at a dictionary.