Octopi is not the correct plural. Octopus is of greek origin, not latin. Thus, its greek plural is octopodes. And thus, since we don't tend to use greek plurals in english, the other proper plural is octopuses.
Octopi is what's known as a hypercorrection. "A hypercorrection is non-standard use of language that results from the over-application of a perceived rule of language-usage prescription. A speaker or writer who produces a hypercorrection generally believes through a misunderstanding of such rules that the form is more "correct", standard, or otherwise preferable, often combined with a desire to appear formal."[1]
Now, you will find octopi in the dictionary, but only because so many people have engaged in this hypercorrection that it has become widespread and dictionaries reflect common usage.