I just learned that octopuses and octopodes are correct plural forms of octopus. Octopi is not correct because it uses a latin plural ending for a greek word, though it is used enough that it is acceptable.
Now that I got to share my cool bit of vocab trivia, we can move right along to the reason I looked that up: crowns! Crowns with octopodes! There are several different octopus and shark crowns listed in my shop right now under the costumes and crowns category: http://www.auggiefroggy.com/shop/#costumes