How do you make octopus plural? It’s simple!
The plural is octopuses. Why do some dictionaries also list octopi as a possibility? In Latin, some plurals end with an i. The problem is, octopus derives from Greek. The i was a mistake, but so many people adopted it that it became an acceptable alternative. Many people don’t like octopi, and you will rarely see it in edited works, but it does occasionally appear.
The plural of octopus is almost as fascinating as the animal. What more can you learn about plurals and spelling?
from Grammarly Blog https://www.grammarly.com/blog/octopi-octopuses/