Octopuses is correct by English rules.
Octopodes is correct by strict etymological Greek-origin rules.
Octopi is correct by English's pseudo-Latin cognate rules.
The latter means that any singular noun ending in a consonant and "-us" can be pluralized by chopping off the "-us" and appending an "-i" or "-ii", like alumnus, hippopotamus, virus, bacillus, hummus, lotus, oculus, cactus, and platypus. This has even been abused to facetiously pluralize similar-sounding endings, like Elvis or Winklevoss.This is likely due to the many Anglophone schoolchildren subjected to Latin language lessons over the centuries who simply never cared about proper Latin grammar rules.