Level 10: all forms are valid as long as enough people use them. The currently most used forms are octopuses and octopi, both valid, but octopi is malformed, so octopuses is preferred. Octopussses and octopii and rare variants of those. Also correct, but rarely used.
Octopodes is also correct, but considered pedantic.
Level 10: all forms are valid as long as enough people use them. The currently most used forms are octopuses and octopi, both valid, but octopi is malformed, so octopuses is preferred. Octopussses and octopii and rare variants of those. Also correct, but rarely used.
Octopodes is also correct, but considered pedantic.
Level 11: Just use what you are used to.
Level ATF&C: Glob