Here are some more words that have unusual plurals.
Appendix => Appendices
Now, according to Merriam-Webster, you can legally use appendixes or appendices. They are equal variants, which means neither is favored over the other in that dictionary. However, many members of the Grammar Gestapo cringe whenever they see “appendixes,” yours truly being one of them. Doctors, however, don’t care in the least, because humans only have one appendix, and many have had it removed. Har-har-har.
Vortex => Vortices
This one also lists vortexes as an appropriate plural, but instead of the word “or” in between the two choices, the word “also” is used. That means that vortices is preferred to vortexes, the latter being a secondary variant that is used less frequently than the former.
Vertex => Vertices
This pair follows the same variant rule as Vortex, in that vertexes is listed as a secondary variant using the word “also” rather than “or.”
Fungus => Fungi
Funguses is listed in the dictionary as a secondary variant, so the word “fungi” is preferred.
Cactus => Cacti
Cactuses is listed as a secondary variant – and surprisingly so is cactus. So I guess cactus can be either singular or plural, like deer. But “cacti” is preferred.
Octopus => Octopi
Octopuses, an equal variant, is listed in the dictionary with “or” separating the two choices. A third possibility is listed as a secondary variant in the Oxford English Dictionary, and that is octopodes.
OK, here is the Question of the Day:
What is the plural of Prius?
Would you believe Toyota held a contest in February 2011 to determine the plural?
Choices included Prien, Prii, Prium, Prius, or Priuses. Well, the most popular choice was Prii, and as of Feb. 20, 2011, that became the official plural for that model of car.
Normally, I do not condone democracy in grammatical matters, but this is a case of grammocracy at its finest.