Some English words borrowed directly from other languages retain the plural form of the other language. For example, fermata is pluralized as fermate, and cirrhosis becomes cirrhoses.
The most familiar examples are words taken directly from Latin, of which there are at least hundreds: persona / personae, matrix / matrices, fulcrum / fulcra, and so forth, but there are examples from Greek, Italian, and so on. Over time, some plurals have been regularized, especially in American English, and especially when referring to modern concepts as opposed to ancient— there are far more database schemas than database schemata out in the wild.
Because the Latin irregular plurals are most common, and sound loftier than the regular plural, there is a tendency to hypercorrect the plural of any word ending in -us and -um in particular, even when there is no historical basis for it. And thus, Oxford tells us
The standard plural in English of octopus is octopuses. However, the word octopus comes from Greek, and the Greek plural form octopodes is still occasionally used. The plural form octopi is mistakenly formed according to rules for Latin plurals, and is therefore incorrect.
At some point, of course, any "incorrect" usage that becomes popular enough becomes "correct," and so it is that various dictionaries will include octopi (and ethoi and diplomae and so on) as acceptable plurals. American Heritage and Merriam-Webster list it as an acceptable alternative. M-W has an Ask the Editor segment on the matter.