From Chris Brooke's site:
"Rhinoceros" is the best one, though, when it comes to plurals.Thesis coming, coming. And check the sidebar for deli feed - it's updated in real time (as opposed to fake time - I'm an expert on this so don't contradict me) and thus a far higher output than when I was actually trying to add content. Ah the ironing.
You can pick from "rhinoceroses", "rhinoceros" (unchanged), "rhinocerotes", "rhinocerons" or "rhinocerontes" (though you can't have "rhinoceri", which is what some people, I think, are taught at school -- along with the equally incorrect "octopi").
I asked a friend once what the plural of "rhinoceros" was, and he immediately answered -- quite correctly -- "rhinos".